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

validator_client/src/initialized_validators.rs

@@ -56,6 +56,8 @@ pub enum Error {
     TokioJoin(tokio::task::JoinError),
     /// There was a filesystem error when deleting a lockfile.
     UnableToDeleteLockfile(io::Error),
+    /// Cannot initialize the same validator twice.
+    DuplicatePublicKey,
 }
 
 /// A method used by a validator to sign messages.
@@ -322,6 +324,42 @@ impl InitializedValidators {
             .map(|v| v.voting_keypair())
     }
 
+    /// Add a validator definition to `self`, overwriting the on-disk representation of `self`.
+    pub async fn add_definition(&mut self, def: ValidatorDefinition) -> Result<(), Error> {
+        if self
+            .definitions
+            .as_slice()
+            .iter()
+            .any(|existing| existing.voting_public_key == def.voting_public_key)
+        {
+            return Err(Error::DuplicatePublicKey);
+        }
+
+        self.definitions.push(def);
+
+        self.update_validators().await?;
+
+        self.definitions
+            .save(&self.validators_dir)
+            .map_err(Error::UnableToSaveDefinitions)?;
+
+        Ok(())
+    }
+
+    /// Returns a slice of all defined validators (regardless of their enabled state).
+    pub fn validator_definitions(&self) -> &[ValidatorDefinition] {
+        self.definitions.as_slice()
+    }
+
+    /// Indicates if the `voting_public_key` exists in self and if it is enabled.
+    pub fn is_enabled(&self, voting_public_key: &PublicKey) -> Option<bool> {
+        self.definitions
+            .as_slice()
+            .iter()
+            .find(|def| def.voting_public_key == *voting_public_key)
+            .map(|def| def.enabled)
+    }
+
     /// Sets the `InitializedValidator` and `ValidatorDefinition` `enabled` values.
     ///
     /// ## Notes