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_keystore/src/keystore.rs

@@ -81,6 +81,7 @@ pub struct KeystoreBuilder<'a> {
     cipher: Cipher,
     uuid: Uuid,
     path: String,
+    description: String,
 }
 
 impl<'a> KeystoreBuilder<'a> {
@@ -105,10 +106,17 @@ impl<'a> KeystoreBuilder<'a> {
                 cipher: Cipher::Aes128Ctr(Aes128Ctr { iv }),
                 uuid: Uuid::new_v4(),
                 path,
+                description: "".to_string(),
             })
         }
     }
 
+    /// Build the keystore with a specific description instead of an empty string.
+    pub fn description(mut self, description: String) -> Self {
+        self.description = description;
+        self
+    }
+
     /// Build the keystore using the supplied `kdf` instead of `crate::default_kdf`.
     pub fn kdf(mut self, kdf: Kdf) -> Self {
         self.kdf = kdf;
@@ -124,6 +132,7 @@ impl<'a> KeystoreBuilder<'a> {
             self.cipher,
             self.uuid,
             self.path,
+            self.description,
         )
     }
 }
@@ -147,6 +156,7 @@ impl Keystore {
         cipher: Cipher,
         uuid: Uuid,
         path: String,
+        description: String,
     ) -> Result<Self, Error> {
         let secret: ZeroizeHash = keypair.sk.serialize();
 
@@ -175,7 +185,7 @@ impl Keystore {
                 path: Some(path),
                 pubkey: keypair.pk.to_hex_string()[2..].to_string(),
                 version: Version::four(),
-                description: None,
+                description: Some(description),
                 name: None,
             },
         })
@@ -228,6 +238,18 @@ impl Keystore {
         &self.json.pubkey
     }
 
+    /// Returns the description for the keystore, if the field is present.
+    pub fn description(&self) -> Option<&str> {
+        self.json.description.as_deref()
+    }
+
+    /// Sets the description for the keystore.
+    ///
+    /// Note: this does not save the keystore to disk.
+    pub fn set_description(&mut self, description: String) {
+        self.json.description = Some(description)
+    }
+
     /// Returns the pubkey for the keystore, parsed as a `PublicKey` if it parses.
     pub fn public_key(&self) -> Option<PublicKey> {
         serde_json::from_str(&format!("\"0x{}\"", &self.json.pubkey)).ok()

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