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

crypto/eth2_wallet/src/wallet.rs

@@ -215,6 +215,23 @@ impl Wallet {
         self.json.nextaccount
     }
 
+    /// Sets the value of the JSON wallet `nextaccount` field.
+    ///
+    /// This will be the index of the next wallet generated with `Self::next_validator`.
+    ///
+    /// ## Errors
+    ///
+    /// Returns `Err(())` if `nextaccount` is less than `self.nextaccount()` without mutating
+    /// `self`. This is to protect against duplicate validator generation.
+    pub fn set_nextaccount(&mut self, nextaccount: u32) -> Result<(), ()> {
+        if nextaccount >= self.nextaccount() {
+            self.json.nextaccount = nextaccount;
+            Ok(())
+        } else {
+            Err(())
+        }
+    }
+
     /// Returns the value of the JSON wallet `name` field.
     pub fn name(&self) -> &str {
         &self.json.name