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/eth2/Cargo.toml

@@ -25,6 +25,7 @@ eth2_ssz_derive = "0.3.0"
 futures-util = "0.3.8"
 futures = "0.3.8"
 store = { path = "../../beacon_node/store", optional = true }
+slashing_protection = { path = "../../validator_client/slashing_protection", optional = true }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 # TODO: update psutil once fix is merged: https://github.com/rust-psutil/rust-psutil/pull/93
@@ -35,4 +36,4 @@ procinfo = { version = "0.4.2", optional = true }
 
 [features]
 default = ["lighthouse"]
-lighthouse = ["proto_array", "psutil", "procinfo", "store"]
+lighthouse = ["proto_array", "psutil", "procinfo", "store", "slashing_protection"]