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.~~

Cargo.lock

@@ -6972,6 +6972,7 @@ dependencies = [
  "parking_lot",
  "rand 0.7.3",
  "rayon",
+ "reqwest",
  "ring",
  "safe_arith",
  "scrypt",
@@ -6990,6 +6991,7 @@ dependencies = [
  "tokio",
  "tree_hash 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "types",
+ "url",
  "validator_dir",
  "warp",
  "warp_utils",
@@ -7295,6 +7297,31 @@ dependencies = [
  "url",
 ]
 
+[[package]]
+name = "web3signer_tests"
+version = "0.1.0"
+dependencies = [
+ "account_utils",
+ "environment",
+ "eth2_keystore",
+ "eth2_network_config",
+ "exit-future",
+ "futures",
+ "reqwest",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "serde_yaml",
+ "slot_clock",
+ "task_executor",
+ "tempfile",
+ "tokio",
+ "types",
+ "url",
+ "validator_client",
+ "zip",
+]
+
 [[package]]
 name = "webpki"
 version = "0.21.4"