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/validator_dir/src/builder.rs

@@ -40,6 +40,7 @@ pub enum Error {
     UninitializedWithdrawalKeystore,
     #[cfg(feature = "insecure_keys")]
     InsecureKeysError(String),
+    MissingPasswordDir,
 }
 
 impl From<KeystoreError> for Error {
@@ -51,7 +52,7 @@ impl From<KeystoreError> for Error {
 /// A builder for creating a `ValidatorDir`.
 pub struct Builder<'a> {
     base_validators_dir: PathBuf,
-    password_dir: PathBuf,
+    password_dir: Option<PathBuf>,
     pub(crate) voting_keystore: Option<(Keystore, PlainText)>,
     pub(crate) withdrawal_keystore: Option<(Keystore, PlainText)>,
     store_withdrawal_keystore: bool,
@@ -60,10 +61,10 @@ pub struct Builder<'a> {
 
 impl<'a> Builder<'a> {
     /// Instantiate a new builder.
-    pub fn new(base_validators_dir: PathBuf, password_dir: PathBuf) -> Self {
+    pub fn new(base_validators_dir: PathBuf) -> Self {
         Self {
             base_validators_dir,
-            password_dir,
+            password_dir: None,
             voting_keystore: None,
             withdrawal_keystore: None,
             store_withdrawal_keystore: true,
@@ -71,6 +72,12 @@ impl<'a> Builder<'a> {
         }
     }
 
+    /// Supply a directory in which to store the passwords for the validator keystores.
+    pub fn password_dir<P: Into<PathBuf>>(mut self, password_dir: P) -> Self {
+        self.password_dir = Some(password_dir.into());
+        self
+    }
+
     /// Build the `ValidatorDir` use the given `keystore` which can be unlocked with `password`.
     ///
     /// The builder will not necessarily check that `password` can unlock `keystore`.
@@ -215,26 +222,35 @@ impl<'a> Builder<'a> {
                 }
             }
 
-            // Only the withdrawal keystore if explicitly required.
-            if self.store_withdrawal_keystore {
-                // Write the withdrawal password to file.
-                write_password_to_file(
-                    self.password_dir
-                        .join(withdrawal_keypair.pk.to_hex_string()),
-                    withdrawal_password.as_bytes(),
-                )?;
+            if self.password_dir.is_none() && self.store_withdrawal_keystore {
+                return Err(Error::MissingPasswordDir);
+            }
 
-                // Write the withdrawal keystore to file.
-                write_keystore_to_file(dir.join(WITHDRAWAL_KEYSTORE_FILE), &withdrawal_keystore)?;
+            if let Some(password_dir) = self.password_dir.as_ref() {
+                // Only the withdrawal keystore if explicitly required.
+                if self.store_withdrawal_keystore {
+                    // Write the withdrawal password to file.
+                    write_password_to_file(
+                        password_dir.join(withdrawal_keypair.pk.to_hex_string()),
+                        withdrawal_password.as_bytes(),
+                    )?;
+
+                    // Write the withdrawal keystore to file.
+                    write_keystore_to_file(
+                        dir.join(WITHDRAWAL_KEYSTORE_FILE),
+                        &withdrawal_keystore,
+                    )?;
+                }
             }
         }
 
-        // Write the voting password to file.
-        write_password_to_file(
-            self.password_dir
-                .join(format!("0x{}", voting_keystore.pubkey())),
-            voting_password.as_bytes(),
-        )?;
+        if let Some(password_dir) = self.password_dir.as_ref() {
+            // Write the voting password to file.
+            write_password_to_file(
+                password_dir.join(format!("0x{}", voting_keystore.pubkey())),
+                voting_password.as_bytes(),
+            )?;
+        }
 
         // Write the voting keystore to file.
         write_keystore_to_file(dir.join(VOTING_KEYSTORE_FILE), &voting_keystore)?;

common/validator_dir/src/insecure_keys.rs

@@ -73,7 +73,8 @@ pub fn build_deterministic_validator_dirs(
     indices: &[usize],
 ) -> Result<(), String> {
     for &i in indices {
-        Builder::new(validators_dir.clone(), password_dir.clone())
+        Builder::new(validators_dir.clone())
+            .password_dir(password_dir.clone())
             .insecure_voting_keypair(i)
             .map_err(|e| format!("Unable to generate insecure keypair: {:?}", e))?
             .store_withdrawal_keystore(false)

common/validator_dir/src/validator_dir.rs

@@ -129,6 +129,11 @@ impl ValidatorDir {
         &self.dir
     }
 
+    /// Returns the path to the voting keystore JSON file.
+    pub fn voting_keystore_path(&self) -> PathBuf {
+        self.dir.join(VOTING_KEYSTORE_FILE)
+    }
+
     /// Attempts to read the keystore in `self.dir` and decrypt the keypair using a password file
     /// in `password_dir`.
     ///