Implement standard keystore API (#2736)

## Issue Addressed

Implements the standard key manager API from https://ethereum.github.io/keymanager-APIs/, formerly https://github.com/ethereum/beacon-APIs/pull/151
Related to https://github.com/sigp/lighthouse/issues/2557

## Proposed Changes

- [x] Add all of the new endpoints from the standard API: GET, POST and DELETE.
- [x] Add a `validators.enabled` column to the slashing protection database to support atomic disable + export.
- [x] Add tests for all the common sequential accesses of the API
- [x] Add tests for interactions with remote signer validators
- [x] Add end-to-end tests for migration of validators from one VC to another
- [x] Implement the authentication scheme from the standard (token bearer auth)

## Additional Info

The `enabled` column in the validators SQL database is necessary to prevent a race condition when exporting slashing protection data. Without the slashing protection database having a way of knowing that a key has been disabled, a concurrent request to sign a message could insert a new record into the database. The `delete_concurrent_with_signing` test exercises this code path, and was indeed failing before the `enabled` column was added.

The validator client authentication has been modified from basic auth to bearer auth, with basic auth preserved for backwards compatibility.

validator_client/src/signing_method.rs

@@ -6,6 +6,7 @@
 use crate::http_metrics::metrics;
 use eth2_keystore::Keystore;
 use lockfile::Lockfile;
+use parking_lot::Mutex;
 use reqwest::Client;
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -75,7 +76,7 @@ pub enum SigningMethod {
     /// A validator that is defined by an EIP-2335 keystore on the local filesystem.
     LocalKeystore {
         voting_keystore_path: PathBuf,
-        voting_keystore_lockfile: Lockfile,
+        voting_keystore_lockfile: Mutex<Option<Lockfile>>,
         voting_keystore: Keystore,
         voting_keypair: Arc<Keypair>,
     },