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/account_utils/src/validator_definitions.rs

@@ -46,9 +46,6 @@ pub enum Error {
 }
 
 /// Defines how the validator client should attempt to sign messages for this validator.
-///
-/// Presently there is only a single variant, however we expect more variants to arise (e.g.,
-/// remote signing).
 #[derive(Clone, PartialEq, Serialize, Deserialize)]
 #[serde(tag = "type")]
 pub enum SigningDefinition {
@@ -78,6 +75,12 @@ pub enum SigningDefinition {
     },
 }
 
+impl SigningDefinition {
+    pub fn is_local_keystore(&self) -> bool {
+        matches!(self, SigningDefinition::LocalKeystore { .. })
+    }
+}
+
 /// A validator that may be initialized by this validator client.
 ///
 /// Presently there is only a single variant, however we expect more variants to arise (e.g.,
@@ -293,6 +296,11 @@ impl ValidatorDefinitions {
         Ok(())
     }
 
+    /// Retain only the definitions matching the given predicate.
+    pub fn retain(&mut self, f: impl FnMut(&ValidatorDefinition) -> bool) {
+        self.0.retain(f);
+    }
+
     /// Adds a new `ValidatorDefinition` to `self`.
     pub fn push(&mut self, def: ValidatorDefinition) {
         self.0.push(def)