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/account_utils/src/validator_definitions.rs

@@ -290,7 +290,7 @@ pub fn recursively_find_voting_keystores<P: AsRef<Path>>(
 }
 
 /// Returns `true` if we should consider the `file_name` to represent a voting keystore.
-fn is_voting_keystore(file_name: &str) -> bool {
+pub fn is_voting_keystore(file_name: &str) -> bool {
     // All formats end with `.json`.
     if !file_name.ends_with(".json") {
         return false;