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/signing_method/web3signer.rs

@@ -0,0 +1,114 @@
+//! Contains the types required to make JSON requests to Web3Signer servers.
+
+use serde::{Deserialize, Serialize};
+use types::*;
+
+#[derive(Debug, PartialEq, Copy, Clone, Serialize)]
+#[serde(rename_all = "SCREAMING_SNAKE_CASE")]
+pub enum MessageType {
+    AggregationSlot,
+    AggregateAndProof,
+    Attestation,
+    BlockV2,
+    Deposit,
+    RandaoReveal,
+    VoluntaryExit,
+    SyncCommitteeMessage,
+    SyncCommitteeSelectionProof,
+    SyncCommitteeContributionAndProof,
+}
+
+#[derive(Debug, PartialEq, Copy, Clone, Serialize)]
+#[serde(rename_all = "SCREAMING_SNAKE_CASE")]
+pub enum ForkName {
+    Phase0,
+    Altair,
+}
+
+#[derive(Debug, PartialEq, Serialize)]
+pub struct ForkInfo {
+    pub fork: Fork,
+    pub genesis_validators_root: Hash256,
+}
+
+#[derive(Debug, PartialEq, Serialize)]
+#[serde(bound = "T: EthSpec", rename_all = "snake_case")]
+pub enum Web3SignerObject<'a, T: EthSpec> {
+    AggregationSlot {
+        slot: Slot,
+    },
+    AggregateAndProof(&'a AggregateAndProof<T>),
+    Attestation(&'a AttestationData),
+    BeaconBlock {
+        version: ForkName,
+        block: &'a BeaconBlock<T>,
+    },
+    #[allow(dead_code)]
+    Deposit {
+        pubkey: PublicKeyBytes,
+        withdrawal_credentials: Hash256,
+        #[serde(with = "eth2_serde_utils::quoted_u64")]
+        amount: u64,
+        #[serde(with = "eth2_serde_utils::bytes_4_hex")]
+        genesis_fork_version: [u8; 4],
+    },
+    RandaoReveal {
+        epoch: Epoch,
+    },
+    #[allow(dead_code)]
+    VoluntaryExit(&'a VoluntaryExit),
+    SyncCommitteeMessage {
+        beacon_block_root: Hash256,
+        slot: Slot,
+    },
+    SyncAggregatorSelectionData(&'a SyncAggregatorSelectionData),
+    ContributionAndProof(&'a ContributionAndProof<T>),
+}
+
+impl<'a, T: EthSpec> Web3SignerObject<'a, T> {
+    pub fn beacon_block(block: &'a BeaconBlock<T>) -> Self {
+        let version = match block {
+            BeaconBlock::Base(_) => ForkName::Phase0,
+            BeaconBlock::Altair(_) => ForkName::Altair,
+        };
+
+        Web3SignerObject::BeaconBlock { version, block }
+    }
+
+    pub fn message_type(&self) -> MessageType {
+        match self {
+            Web3SignerObject::AggregationSlot { .. } => MessageType::AggregationSlot,
+            Web3SignerObject::AggregateAndProof(_) => MessageType::AggregateAndProof,
+            Web3SignerObject::Attestation(_) => MessageType::Attestation,
+            Web3SignerObject::BeaconBlock { .. } => MessageType::BlockV2,
+            Web3SignerObject::Deposit { .. } => MessageType::Deposit,
+            Web3SignerObject::RandaoReveal { .. } => MessageType::RandaoReveal,
+            Web3SignerObject::VoluntaryExit(_) => MessageType::VoluntaryExit,
+            Web3SignerObject::SyncCommitteeMessage { .. } => MessageType::SyncCommitteeMessage,
+            Web3SignerObject::SyncAggregatorSelectionData(_) => {
+                MessageType::SyncCommitteeSelectionProof
+            }
+            Web3SignerObject::ContributionAndProof(_) => {
+                MessageType::SyncCommitteeContributionAndProof
+            }
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Serialize)]
+#[serde(bound = "T: EthSpec")]
+pub struct SigningRequest<'a, T: EthSpec> {
+    #[serde(rename = "type")]
+    pub message_type: MessageType,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub fork_info: Option<ForkInfo>,
+    #[serde(rename = "signingRoot")]
+    pub signing_root: Hash256,
+    #[serde(flatten)]
+    pub object: Web3SignerObject<'a, T>,
+}
+
+#[derive(Debug, PartialEq, Deserialize)]
+pub struct SigningResponse {
+    pub signature: Signature,
+}