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/lib.rs

@@ -2,7 +2,10 @@
 //! Lighthouse project.
 
 use eth2_keystore::Keystore;
-use eth2_wallet::Wallet;
+use eth2_wallet::{
+    bip39::{Language, Mnemonic, MnemonicType},
+    Wallet,
+};
 use rand::{distributions::Alphanumeric, Rng};
 use serde_derive::{Deserialize, Serialize};
 use std::fs::{self, File};
@@ -15,6 +18,7 @@ use zeroize::Zeroize;
 pub mod validator_definitions;
 
 pub use eth2_keystore;
+pub use eth2_wallet;
 pub use eth2_wallet::PlainText;
 
 /// The minimum number of characters required for a wallet password.
@@ -150,6 +154,16 @@ pub fn is_password_sufficiently_complex(password: &[u8]) -> Result<(), String> {
     }
 }
 
+/// Returns a random 24-word english mnemonic.
+pub fn random_mnemonic() -> Mnemonic {
+    Mnemonic::new(MnemonicType::Words24, Language::English)
+}
+
+/// Attempts to parse a mnemonic phrase.
+pub fn mnemonic_from_phrase(phrase: &str) -> Result<Mnemonic, String> {
+    Mnemonic::from_phrase(phrase, Language::English).map_err(|e| e.to_string())
+}
+
 /// Provides a new-type wrapper around `String` that is zeroized on `Drop`.
 ///
 /// Useful for ensuring that password memory is zeroed-out on drop.
@@ -164,6 +178,12 @@ impl From<String> for ZeroizeString {
     }
 }
 
+impl ZeroizeString {
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
 impl AsRef<[u8]> for ZeroizeString {
     fn as_ref(&self) -> &[u8] {
         self.0.as_bytes()

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,