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/create_validator.rs

@@ -1,4 +1,5 @@
 use crate::ValidatorStore;
+use account_utils::validator_definitions::{SigningDefinition, ValidatorDefinition};
 use account_utils::{
     eth2_wallet::{bip39::Mnemonic, WalletBuilder},
     random_mnemonic, random_password, ZeroizeString,
@@ -21,7 +22,7 @@ use validator_dir::Builder as ValidatorDirBuilder;
 ///
 /// If `key_derivation_path_offset` is supplied then the EIP-2334 validator index will start at
 /// this point.
-pub async fn create_validators<P: AsRef<Path>, T: 'static + SlotClock, E: EthSpec>(
+pub async fn create_validators_mnemonic<P: AsRef<Path>, T: 'static + SlotClock, E: EthSpec>(
     mnemonic_opt: Option<Mnemonic>,
     key_derivation_path_offset: Option<u32>,
     validator_requests: &[api_types::ValidatorRequest],
@@ -159,3 +160,33 @@ pub async fn create_validators<P: AsRef<Path>, T: 'static + SlotClock, E: EthSpe
 
     Ok((validators, mnemonic))
 }
+
+pub async fn create_validators_web3signer<T: 'static + SlotClock, E: EthSpec>(
+    validator_requests: &[api_types::Web3SignerValidatorRequest],
+    validator_store: &ValidatorStore<T, E>,
+) -> Result<(), warp::Rejection> {
+    for request in validator_requests {
+        let validator_definition = ValidatorDefinition {
+            enabled: request.enable,
+            voting_public_key: request.voting_public_key.clone(),
+            graffiti: request.graffiti.clone(),
+            description: request.description.clone(),
+            signing_definition: SigningDefinition::Web3Signer {
+                url: request.url.clone(),
+                root_certificate_path: request.root_certificate_path.clone(),
+                request_timeout_ms: request.request_timeout_ms,
+            },
+        };
+        validator_store
+            .add_validator(validator_definition)
+            .await
+            .map_err(|e| {
+                warp_utils::reject::custom_server_error(format!(
+                    "failed to initialize validator: {:?}",
+                    e
+                ))
+            })?;
+    }
+
+    Ok(())
+}