Add cli option for voluntary exits (#1781)

## Issue Addressed

Resolve #1652 

## Proposed Changes

Adds a cli option for voluntary exits. The flow is similar to prysm's where after entering the password for the validator keystore (or load password from `secrets` if present) the user is given multiple warnings about the operation being irreversible, then redirected to the docs webpage(not added yet) which explains what a voluntary exit is and the consequences of exiting and then prompted to enter a phrase from the docs webpage as a final confirmation. 

Example usage
```
$ lighthouse --testnet zinken account validator exit --validator <validator-pubkey> --beacon-node http://localhost:5052

Running account manager for zinken testnet                                                                                                          
validator-dir path: "..."

Enter the keystore password:  for validator in ...

Password is correct

Publishing a voluntary exit for validator: ...              
WARNING: This is an irreversible operation                                                                                                                    
WARNING: Withdrawing staked eth will not be possible until Eth1/Eth2 merge Please visit [website] to make sure you understand the implications of a voluntary exit.            
                                                                                                                                             
Enter the phrase from the above URL to confirm the voluntary exit:
Exit my validator
Published voluntary exit for validator ...
```

## Additional info

Not sure if we should have batch exits (`--validator all`) option for exiting all the validators in the `validators` directory. I'm slightly leaning towards having only single exits but don't have a strong preference.

common/validator_dir/src/lib.rs

@@ -12,7 +12,10 @@ pub mod insecure_keys;
 mod manager;
 mod validator_dir;
 
-pub use crate::validator_dir::{Error, Eth1DepositData, ValidatorDir, ETH1_DEPOSIT_TX_HASH_FILE};
+pub use crate::validator_dir::{
+    unlock_keypair_from_password_path, Error, Eth1DepositData, ValidatorDir,
+    ETH1_DEPOSIT_TX_HASH_FILE,
+};
 pub use builder::{
     Builder, Error as BuilderError, ETH1_DEPOSIT_DATA_FILE, VOTING_KEYSTORE_FILE,
     WITHDRAWAL_KEYSTORE_FILE,

common/validator_dir/src/validator_dir.rs

@@ -143,7 +143,7 @@ impl ValidatorDir {
     ///
     /// If there is a filesystem error, a password is missing or the password is incorrect.
     pub fn voting_keypair<P: AsRef<Path>>(&self, password_dir: P) -> Result<Keypair, Error> {
-        unlock_keypair(&self.dir.clone(), VOTING_KEYSTORE_FILE, password_dir)
+        unlock_keypair(&self.dir.join(VOTING_KEYSTORE_FILE), password_dir)
     }
 
     /// Attempts to read the keystore in `self.dir` and decrypt the keypair using a password file
@@ -155,7 +155,7 @@ impl ValidatorDir {
     ///
     /// If there is a file-system error, a password is missing or the password is incorrect.
     pub fn withdrawal_keypair<P: AsRef<Path>>(&self, password_dir: P) -> Result<Keypair, Error> {
-        unlock_keypair(&self.dir.clone(), WITHDRAWAL_KEYSTORE_FILE, password_dir)
+        unlock_keypair(&self.dir.join(WITHDRAWAL_KEYSTORE_FILE), password_dir)
     }
 
     /// Indicates if there is a file containing an eth1 deposit transaction. This can be used to
@@ -250,17 +250,16 @@ impl Drop for ValidatorDir {
     }
 }
 
-/// Attempts to load and decrypt a keystore.
-fn unlock_keypair<P: AsRef<Path>>(
-    keystore_dir: &PathBuf,
-    filename: &str,
+/// Attempts to load and decrypt a Keypair given path to the keystore.
+pub fn unlock_keypair<P: AsRef<Path>>(
+    keystore_path: &PathBuf,
     password_dir: P,
 ) -> Result<Keypair, Error> {
     let keystore = Keystore::from_json_reader(
         &mut OpenOptions::new()
             .read(true)
             .create(false)
-            .open(keystore_dir.clone().join(filename))
+            .open(keystore_path)
             .map_err(Error::UnableToOpenKeystore)?,
     )
     .map_err(Error::UnableToReadKeystore)?;
@@ -271,7 +270,28 @@ fn unlock_keypair<P: AsRef<Path>>(
     let password: PlainText = read(&password_path)
         .map_err(|_| Error::UnableToReadPassword(password_path))?
         .into();
-
+    keystore
+        .decrypt_keypair(password.as_bytes())
+        .map_err(Error::UnableToDecryptKeypair)
+}
+
+/// Attempts to load and decrypt a Keypair given path to the keystore and the password file.
+pub fn unlock_keypair_from_password_path(
+    keystore_path: &PathBuf,
+    password_path: &PathBuf,
+) -> Result<Keypair, Error> {
+    let keystore = Keystore::from_json_reader(
+        &mut OpenOptions::new()
+            .read(true)
+            .create(false)
+            .open(keystore_path)
+            .map_err(Error::UnableToOpenKeystore)?,
+    )
+    .map_err(Error::UnableToReadKeystore)?;
+
+    let password: PlainText = read(password_path)
+        .map_err(|_| Error::UnableToReadPassword(password_path.clone()))?
+        .into();
     keystore
         .decrypt_keypair(password.as_bytes())
         .map_err(Error::UnableToDecryptKeypair)