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/Cargo.toml

@@ -10,6 +10,7 @@ path = "src/lib.rs"
 
 [dev-dependencies]
 tokio = { version = "1.14.0", features = ["time", "rt-multi-thread", "macros"] }
+logging = { path = "../common/logging" }
 
 [dependencies]
 tree_hash = "0.4.1"
@@ -48,7 +49,7 @@ hyper = "0.14.4"
 eth2_serde_utils = "0.1.1"
 libsecp256k1 = "0.6.0"
 ring = "0.16.19"
-rand = "0.7.3"
+rand = { version = "0.7.3", features = ["small_rng"] }
 lighthouse_metrics = { path = "../common/lighthouse_metrics" }
 lazy_static = "1.4.0"
 itertools = "0.10.0"