Web3Signer support for VC (#2522)

[EIP-3030]: https://eips.ethereum.org/EIPS/eip-3030
[Web3Signer]: https://consensys.github.io/web3signer/web3signer-eth2.html

## Issue Addressed

Resolves #2498

## Proposed Changes

Allows the VC to call out to a [Web3Signer] remote signer to obtain signatures.


## Additional Info

### Making Signing Functions `async`

To allow remote signing, I needed to make all the signing functions `async`. This caused a bit of noise where I had to convert iterators into `for` loops.

In `duties_service.rs` there was a particularly tricky case where we couldn't hold a write-lock across an `await`, so I had to first take a read-lock, then grab a write-lock.

### Move Signing from Core Executor

Whilst implementing this feature, I noticed that we signing was happening on the core tokio executor. I suspect this was causing the executor to temporarily lock and occasionally trigger some HTTP timeouts (and potentially SQL pool timeouts, but I can't verify this). Since moving all signing into blocking tokio tasks, I noticed a distinct drop in the "atttestations_http_get" metric on a Prater node:

![http_get_times](https://user-images.githubusercontent.com/6660660/132143737-82fd3836-2e7e-445b-a143-cb347783baad.png)

I think this graph indicates that freeing the core executor allows the VC to operate more smoothly.

### Refactor TaskExecutor

I noticed that the `TaskExecutor::spawn_blocking_handle` function would fail to spawn tasks if it were unable to obtain handles to some metrics (this can happen if the same metric is defined twice). It seemed that a more sensible approach would be to keep spawning tasks, but without metrics. To that end, I refactored the function so that it would still function without metrics. There are no other changes made.

## TODO

- [x] Restructure to support multiple signing methods.
- [x] Add calls to remote signer from VC.
- [x] Documentation
- [x] Test all endpoints
- [x] Test HTTPS certificate
- [x] Allow adding remote signer validators via the API
- [x] Add Altair support via [21.8.1-rc1](https://github.com/ConsenSys/web3signer/releases/tag/21.8.1-rc1)
- [x] Create issue to start using latest version of web3signer. (See #2570)

## Notes

- ~~Web3Signer doesn't yet support the Altair fork for Prater. See https://github.com/ConsenSys/web3signer/issues/423.~~
- ~~There is not yet a release of Web3Signer which supports Altair blocks. See https://github.com/ConsenSys/web3signer/issues/391.~~

validator_client/src/http_api/mod.rs

@@ -4,7 +4,7 @@ mod tests;
 
 use crate::ValidatorStore;
 use account_utils::mnemonic_from_phrase;
-use create_validator::create_validators;
+use create_validator::{create_validators_mnemonic, create_validators_web3signer};
 use eth2::lighthouse_vc::types::{self as api_types, PublicKey, PublicKeyBytes};
 use lighthouse_version::version_with_platform;
 use serde::{Deserialize, Serialize};
@@ -273,14 +273,15 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
              runtime: Weak<Runtime>| {
                 blocking_signed_json_task(signer, move || {
                     if let Some(runtime) = runtime.upgrade() {
-                        let (validators, mnemonic) = runtime.block_on(create_validators(
-                            None,
-                            None,
-                            &body,
-                            &validator_dir,
-                            &validator_store,
-                            &spec,
-                        ))?;
+                        let (validators, mnemonic) =
+                            runtime.block_on(create_validators_mnemonic(
+                                None,
+                                None,
+                                &body,
+                                &validator_dir,
+                                &validator_store,
+                                &spec,
+                            ))?;
                         let response = api_types::PostValidatorsResponseData {
                             mnemonic: mnemonic.into_phrase().into(),
                             validators,
@@ -322,14 +323,15 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                                     e
                                 ))
                             })?;
-                        let (validators, _mnemonic) = runtime.block_on(create_validators(
-                            Some(mnemonic),
-                            Some(body.key_derivation_path_offset),
-                            &body.validators,
-                            &validator_dir,
-                            &validator_store,
-                            &spec,
-                        ))?;
+                        let (validators, _mnemonic) =
+                            runtime.block_on(create_validators_mnemonic(
+                                Some(mnemonic),
+                                Some(body.key_derivation_path_offset),
+                                &body.validators,
+                                &validator_dir,
+                                &validator_store,
+                                &spec,
+                            ))?;
                         Ok(api_types::GenericResponse::from(validators))
                     } else {
                         Err(warp_utils::reject::custom_server_error(
@@ -416,6 +418,33 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
             },
         );
 
+    // POST lighthouse/validators/web3signer
+    let post_validators_web3signer = warp::path("lighthouse")
+        .and(warp::path("validators"))
+        .and(warp::path("web3signer"))
+        .and(warp::path::end())
+        .and(warp::body::json())
+        .and(validator_store_filter.clone())
+        .and(signer.clone())
+        .and(runtime_filter.clone())
+        .and_then(
+            |body: Vec<api_types::Web3SignerValidatorRequest>,
+             validator_store: Arc<ValidatorStore<T, E>>,
+             signer,
+             runtime: Weak<Runtime>| {
+                blocking_signed_json_task(signer, move || {
+                    if let Some(runtime) = runtime.upgrade() {
+                        runtime.block_on(create_validators_web3signer(&body, &validator_store))?;
+                        Ok(())
+                    } else {
+                        Err(warp_utils::reject::custom_server_error(
+                            "Runtime shutdown".into(),
+                        ))
+                    }
+                })
+            },
+        );
+
     // PATCH lighthouse/validators/{validator_pubkey}
     let patch_validators = warp::path("lighthouse")
         .and(warp::path("validators"))
@@ -484,7 +513,8 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                 .or(warp::post().and(
                     post_validators
                         .or(post_validators_keystore)
-                        .or(post_validators_mnemonic),
+                        .or(post_validators_mnemonic)
+                        .or(post_validators_web3signer),
                 ))
                 .or(warp::patch().and(patch_validators)),
         )