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/account_utils/src/validator_definitions.rs

@@ -63,6 +63,8 @@ pub enum SigningDefinition {
 pub struct ValidatorDefinition {
     pub enabled: bool,
     pub voting_public_key: PublicKey,
+    #[serde(default)]
+    pub description: String,
     #[serde(flatten)]
     pub signing_definition: SigningDefinition,
 }
@@ -88,6 +90,7 @@ impl ValidatorDefinition {
         Ok(ValidatorDefinition {
             enabled: true,
             voting_public_key,
+            description: keystore.description().unwrap_or_else(|| "").to_string(),
             signing_definition: SigningDefinition::LocalKeystore {
                 voting_keystore_path,
                 voting_keystore_password_path: None,
@@ -205,6 +208,7 @@ impl ValidatorDefinitions {
                 Some(ValidatorDefinition {
                     enabled: true,
                     voting_public_key,
+                    description: keystore.description().unwrap_or_else(|| "").to_string(),
                     signing_definition: SigningDefinition::LocalKeystore {
                         voting_keystore_path,
                         voting_keystore_password_path,