Implement VC API (#1657)

## Issue Addressed

NA

## Proposed Changes

- Implements a HTTP API for the validator client.
- Creates EIP-2335 keystores with an empty `description` field, instead of a missing `description` field. Adds option to set name.
- Be more graceful with setups without any validators (yet)
    - Remove an error log when there are no validators.
    - Create the `validator` dir if it doesn't exist.
- Allow building a `ValidatorDir` without a withdrawal keystore (required for the API method where we only post a voting keystore).
- Add optional `description` field to `validator_definitions.yml`

## TODO

- [x] Signature header, as per https://github.com/sigp/lighthouse/issues/1269#issuecomment-649879855
- [x] Return validator descriptions
- [x] Return deposit data
- [x] Respect the mnemonic offset
- [x] Check that mnemonic can derive returned keys
- [x] Be strict about non-localhost
- [x] Allow graceful start without any validators (+ create validator dir)
- [x] Docs final pass
- [x] Swap to EIP-2335 description field. 
- [x] Fix Zerioze TODO in VC api types.
- [x] Zeroize secp256k1 key

## Endpoints

- [x] `GET /lighthouse/version`
- [x] `GET /lighthouse/health`
- [x] `GET /lighthouse/validators` 
- [x] `POST /lighthouse/validators/hd`
- [x] `POST /lighthouse/validators/keystore`
- [x] `PATCH /lighthouse/validators/:validator_pubkey`
- [ ] ~~`POST /lighthouse/validators/:validator_pubkey/exit/:epoch`~~ Future works


## Additional Info

TBC

common/warp_utils/src/reject.rs

@@ -101,6 +101,15 @@ pub fn not_synced(msg: String) -> warp::reject::Rejection {
     warp::reject::custom(NotSynced(msg))
 }
 
+#[derive(Debug)]
+pub struct InvalidAuthorization(pub String);
+
+impl Reject for InvalidAuthorization {}
+
+pub fn invalid_auth(msg: String) -> warp::reject::Rejection {
+    warp::reject::custom(InvalidAuthorization(msg))
+}
+
 /// This function receives a `Rejection` and tries to return a custom
 /// value, otherwise simply passes the rejection along.
 pub async fn handle_rejection(err: warp::Rejection) -> Result<impl warp::Reply, Infallible> {
@@ -150,6 +159,15 @@ pub async fn handle_rejection(err: warp::Rejection) -> Result<impl warp::Reply,
     } else if let Some(e) = err.find::<crate::reject::NotSynced>() {
         code = StatusCode::SERVICE_UNAVAILABLE;
         message = format!("SERVICE_UNAVAILABLE: beacon node is syncing: {}", e.0);
+    } else if let Some(e) = err.find::<crate::reject::InvalidAuthorization>() {
+        code = StatusCode::FORBIDDEN;
+        message = format!("FORBIDDEN: Invalid auth token: {}", e.0);
+    } else if let Some(e) = err.find::<warp::reject::MissingHeader>() {
+        code = StatusCode::BAD_REQUEST;
+        message = format!("BAD_REQUEST: missing {} header", e.name());
+    } else if let Some(e) = err.find::<warp::reject::InvalidHeader>() {
+        code = StatusCode::BAD_REQUEST;
+        message = format!("BAD_REQUEST: invalid {} header", e.name());
     } else if err.find::<warp::reject::MethodNotAllowed>().is_some() {
         code = StatusCode::METHOD_NOT_ALLOWED;
         message = "METHOD_NOT_ALLOWED".to_string();