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_metrics/metrics.rs

@@ -30,6 +30,8 @@ pub const VALIDATOR_ID_HTTP_GET: &str = "validator_id_http_get";
 pub const SUBSCRIPTIONS_HTTP_POST: &str = "subscriptions_http_post";
 pub const UPDATE_PROPOSERS: &str = "update_proposers";
 pub const SUBSCRIPTIONS: &str = "subscriptions";
+pub const LOCAL_KEYSTORE: &str = "local_keystore";
+pub const WEB3SIGNER: &str = "web3signer";
 
 pub use lighthouse_metrics::*;
 
@@ -138,6 +140,14 @@ lazy_static::lazy_static! {
         "sync_eth2_fallback_connected",
         "Set to 1 if connected to atleast one synced eth2 fallback node, otherwise set to 0",
     );
+    /*
+     * Signing Metrics
+     */
+    pub static ref SIGNING_TIMES: Result<HistogramVec> = try_create_histogram_vec(
+        "vc_signing_times_seconds",
+        "Duration to obtain a signature",
+        &["type"]
+    );
 }
 
 pub fn gather_prometheus_metrics<T: EthSpec>(