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.

account_manager/src/validator/slashing_protection.rs

@@ -44,6 +44,7 @@ pub fn cli_run<T: EthSpec>(
     env: Environment<T>,
     validator_base_dir: PathBuf,
 ) -> Result<(), String> {
+    eprintln!("validator-dir path: {:?}", validator_base_dir);
     let slashing_protection_db_path = validator_base_dir.join(SLASHING_PROTECTION_FILENAME);
 
     let testnet_config = env