Mnemonic key recovery (#1579)

## Issue Addressed

N/A

## Proposed Changes

Add a  `lighthouse am wallet recover` command that recreates a wallet from a mnemonic but no validator keys.  Add a `lighthouse am validator recover` command which would directly create keys from a mnemonic for a given index and count.

## Additional Info


Co-authored-by: Paul Hauner <paul@paulhauner.com>

crypto/eth2_wallet/src/lib.rs

@@ -6,6 +6,6 @@ pub mod json_wallet;
 pub use bip39;
 pub use validator_path::{KeyType, ValidatorPath, COIN_TYPE, PURPOSE};
 pub use wallet::{
-    recover_validator_secret, DerivedKey, Error, KeystoreError, PlainText, Uuid,
-    ValidatorKeystores, Wallet, WalletBuilder,
+    recover_validator_secret, recover_validator_secret_from_mnemonic, DerivedKey, Error,
+    KeystoreError, PlainText, Uuid, ValidatorKeystores, Wallet, WalletBuilder,
 };

crypto/eth2_wallet/src/wallet.rs

@@ -285,3 +285,19 @@ pub fn recover_validator_secret(
 
     Ok((destination.secret().to_vec().into(), path))
 }
+
+/// Returns `(secret, path)` for the `key_type` for the validator at `index`.
+///
+/// This function should only be used for key recovery since it can easily lead to key duplication.
+pub fn recover_validator_secret_from_mnemonic(
+    secret: &[u8],
+    index: u32,
+    key_type: KeyType,
+) -> Result<(PlainText, ValidatorPath), Error> {
+    let path = ValidatorPath::new(index, key_type);
+    let master = DerivedKey::from_seed(secret).map_err(|()| Error::EmptyPassword)?;
+
+    let destination = path.iter_nodes().fold(master, |dk, i| dk.child(*i));
+
+    Ok((destination.secret().to_vec().into(), path))
+}