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.

consensus/serde_utils/Cargo.toml

@@ -9,8 +9,6 @@ license = "Apache-2.0"
 [dependencies]
 serde = { version = "1.0.116", features = ["derive"] }
 serde_derive = "1.0.116"
+serde_json = "1.0.58"
 hex = "0.4.2"
 ethereum-types = "0.12.1"
-
-[dev-dependencies]
-serde_json = "1.0.58"

consensus/serde_utils/src/json_str.rs

@@ -0,0 +1,25 @@
+//! Serialize a datatype as a JSON-blob within a single string.
+use serde::{
+    de::{DeserializeOwned, Error as _},
+    ser::Error as _,
+    Deserialize, Deserializer, Serialize, Serializer,
+};
+
+/// Serialize as a JSON object within a string.
+pub fn serialize<T, S>(value: &T, serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+    T: Serialize,
+{
+    serializer.serialize_str(&serde_json::to_string(value).map_err(S::Error::custom)?)
+}
+
+/// Deserialize a JSON object embedded in a string.
+pub fn deserialize<'de, T, D>(deserializer: D) -> Result<T, D::Error>
+where
+    D: Deserializer<'de>,
+    T: DeserializeOwned,
+{
+    let json_str = String::deserialize(deserializer)?;
+    serde_json::from_str(&json_str).map_err(D::Error::custom)
+}

consensus/serde_utils/src/lib.rs

@@ -3,6 +3,7 @@ mod quoted_int;
 pub mod fixed_bytes_hex;
 pub mod hex;
 pub mod hex_vec;
+pub mod json_str;
 pub mod list_of_bytes_lists;
 pub mod quoted_u64_vec;
 pub mod u32_hex;