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/eth2/src/lib.rs

@@ -9,6 +9,7 @@
 
 #[cfg(feature = "lighthouse")]
 pub mod lighthouse;
+pub mod lighthouse_vc;
 pub mod types;
 
 use self::types::*;
@@ -30,6 +31,14 @@ pub enum Error {
     StatusCode(StatusCode),
     /// The supplied URL is badly formatted. It should look something like `http://127.0.0.1:5052`.
     InvalidUrl(Url),
+    /// The supplied validator client secret is invalid.
+    InvalidSecret(String),
+    /// The server returned a response with an invalid signature. It may be an impostor.
+    InvalidSignatureHeader,
+    /// The server returned a response without a signature header. It may be an impostor.
+    MissingSignatureHeader,
+    /// The server returned an invalid JSON response.
+    InvalidJson(serde_json::Error),
 }
 
 impl Error {
@@ -40,6 +49,10 @@ impl Error {
             Error::ServerMessage(msg) => StatusCode::try_from(msg.code).ok(),
             Error::StatusCode(status) => Some(*status),
             Error::InvalidUrl(_) => None,
+            Error::InvalidSecret(_) => None,
+            Error::InvalidSignatureHeader => None,
+            Error::MissingSignatureHeader => None,
+            Error::InvalidJson(_) => None,
         }
     }
 }
@@ -531,7 +544,7 @@ impl BeaconNodeHttpClient {
         self.get(path).await
     }
 
-    /// `GET config/fork_schedule`
+    /// `GET config/spec`
     pub async fn get_config_spec(&self) -> Result<GenericResponse<YamlConfig>, Error> {
         let mut path = self.eth_path()?;