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.~~

book/src/api-vc-endpoints.md

@@ -434,3 +434,43 @@ Typical Responses | 200
     ]
 }
 ```
+
+## `POST /lighthouse/validators/web3signer`
+
+Create any number of new validators, all of which will refer to a
+[Web3Signer](https://docs.web3signer.consensys.net/en/latest/) server for signing.
+
+### HTTP Specification
+
+| Property | Specification |
+| --- |--- |
+Path | `/lighthouse/validators/web3signer`
+Method | POST
+Required Headers | [`Authorization`](./api-vc-auth-header.md)
+Typical Responses | 200, 400
+
+### Example Request Body
+
+```json
+[
+    {
+        "enable": true,
+        "description": "validator_one",
+        "graffiti": "Mr F was here",
+        "voting_public_key": "0xa062f95fee747144d5e511940624bc6546509eeaeae9383257a9c43e7ddc58c17c2bab4ae62053122184c381b90db380",
+        "url": "http://path-to-web3signer.com",
+        "root_certificate_path": "/path/on/vc/filesystem/to/certificate.pem",
+        "request_timeout_ms": 12000
+    }
+]
+```
+
+The following fields may be omitted or nullified to obtain default values:
+
+- `graffiti`
+- `root_certificate_path`
+- `request_timeout_ms`
+
+### Example Response Body
+
+*No data is included in the response body.*