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/Cargo.toml

@@ -15,6 +15,12 @@ reqwest = { version = "0.10.8", features = ["json"] }
 eth2_libp2p = { path = "../../beacon_node/eth2_libp2p" }
 proto_array = { path = "../../consensus/proto_array", optional = true }
 serde_utils = { path = "../../consensus/serde_utils" }
+zeroize = { version = "1.0.0", features = ["zeroize_derive"] }
+eth2_keystore = { path = "../../crypto/eth2_keystore" }
+libsecp256k1 = "0.3.5"
+ring = "0.16.12"
+bytes = "0.5.6"
+account_utils = { path = "../../common/account_utils" }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 psutil = { version = "3.1.0", optional = true }