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.

common/validator_dir/src/builder.rs

@@ -134,15 +134,18 @@ impl<'a> Builder<'a> {
         self
     }
 
+    /// Return the path to the validator dir to be built, i.e. `base_dir/pubkey`.
+    pub fn get_dir_path(base_validators_dir: &Path, voting_keystore: &Keystore) -> PathBuf {
+        base_validators_dir.join(format!("0x{}", voting_keystore.pubkey()))
+    }
+
     /// Consumes `self`, returning a `ValidatorDir` if no error is encountered.
     pub fn build(self) -> Result<ValidatorDir, Error> {
         let (voting_keystore, voting_password) = self
             .voting_keystore
             .ok_or(Error::UninitializedVotingKeystore)?;
 
-        let dir = self
-            .base_validators_dir
-            .join(format!("0x{}", voting_keystore.pubkey()));
+        let dir = Self::get_dir_path(&self.base_validators_dir, &voting_keystore);
 
         if dir.exists() {
             return Err(Error::DirectoryAlreadyExists(dir));