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/src/lib.rs

@@ -28,6 +28,7 @@ use serde::{de::DeserializeOwned, Serialize};
 use std::convert::TryFrom;
 use std::fmt;
 use std::iter::Iterator;
+use std::path::PathBuf;
 use std::time::Duration;
 
 pub const V1: EndpointVersion = EndpointVersion(1);
@@ -59,6 +60,12 @@ pub enum Error {
     InvalidServerSentEvent(String),
     /// The server returned an invalid SSZ response.
     InvalidSsz(ssz::DecodeError),
+    /// An I/O error occurred while loading an API token from disk.
+    TokenReadError(PathBuf, std::io::Error),
+    /// The client has been configured without a server pubkey, but requires one for this request.
+    NoServerPubkey,
+    /// The client has been configured without an API token, but requires one for this request.
+    NoToken,
 }
 
 impl From<reqwest::Error> for Error {
@@ -82,6 +89,8 @@ impl Error {
             Error::InvalidJson(_) => None,
             Error::InvalidServerSentEvent(_) => None,
             Error::InvalidSsz(_) => None,
+            Error::TokenReadError(..) => None,
+            Error::NoServerPubkey | Error::NoToken => None,
         }
     }
 }