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.~~

consensus/types/src/attestation.rs

@@ -9,7 +9,7 @@ use crate::{test_utils::TestRandom, Hash256, Slot};
 
 use super::{
     AggregateSignature, AttestationData, BitList, ChainSpec, Domain, EthSpec, Fork, SecretKey,
-    SignedRoot,
+    Signature, SignedRoot,
 };
 
 #[derive(Debug, PartialEq)]
@@ -60,6 +60,25 @@ impl<T: EthSpec> Attestation<T> {
         fork: &Fork,
         genesis_validators_root: Hash256,
         spec: &ChainSpec,
+    ) -> Result<(), Error> {
+        let domain = spec.get_domain(
+            self.data.target.epoch,
+            Domain::BeaconAttester,
+            fork,
+            genesis_validators_root,
+        );
+        let message = self.data.signing_root(domain);
+
+        self.add_signature(&secret_key.sign(message), committee_position)
+    }
+
+    /// Adds `signature` to `self` and sets the `committee_position`'th bit of `aggregation_bits` to `true`.
+    ///
+    /// Returns an `AlreadySigned` error if the `committee_position`'th bit is already `true`.
+    pub fn add_signature(
+        &mut self,
+        signature: &Signature,
+        committee_position: usize,
     ) -> Result<(), Error> {
         if self
             .aggregation_bits
@@ -72,15 +91,7 @@ impl<T: EthSpec> Attestation<T> {
                 .set(committee_position, true)
                 .map_err(Error::SszTypesError)?;
 
-            let domain = spec.get_domain(
-                self.data.target.epoch,
-                Domain::BeaconAttester,
-                fork,
-                genesis_validators_root,
-            );
-            let message = self.data.signing_root(domain);
-
-            self.signature.add_assign(&secret_key.sign(message));
+            self.signature.add_assign(signature);
 
             Ok(())
         }

consensus/types/src/sync_aggregator_selection_data.rs

@@ -12,6 +12,7 @@ use tree_hash_derive::TreeHash;
 )]
 pub struct SyncAggregatorSelectionData {
     pub slot: Slot,
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub subcommittee_index: u64,
 }