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

beacon_node/http_api/src/lib.rs

@@ -42,6 +42,7 @@ use types::{
     SignedBeaconBlock, SignedVoluntaryExit, Slot, YamlConfig,
 };
 use warp::Filter;
+use warp_utils::task::{blocking_json_task, blocking_task};
 
 const API_PREFIX: &str = "eth";
 const API_VERSION: &str = "v1";
@@ -1727,23 +1728,3 @@ fn publish_network_message<T: EthSpec>(
         ))
     })
 }
-
-/// Execute some task in a tokio "blocking thread". These threads are ideal for long-running
-/// (blocking) tasks since they don't jam up the core executor.
-async fn blocking_task<F, T>(func: F) -> T
-where
-    F: Fn() -> T,
-{
-    tokio::task::block_in_place(func)
-}
-
-/// A convenience wrapper around `blocking_task` for use with `warp` JSON responses.
-async fn blocking_json_task<F, T>(func: F) -> Result<warp::reply::Json, warp::Rejection>
-where
-    F: Fn() -> Result<T, warp::Rejection>,
-    T: Serialize,
-{
-    blocking_task(func)
-        .await
-        .map(|resp| warp::reply::json(&resp))
-}