Add ability to generate signed exit messages with mnemonic or validator key

[valefar-on-discord]

Port from ethereum#355

Adds two commands:
exit-transaction-keystore and exit-transaction-mnemonic

exit-transaction-keystore

Allows the user to specify a single keystore file to generate an exit message if the provided password is correct.
The success of this command will result in a single signed_exit_transaction-***.json file in exit_transactions

exit-transaction-mnemonic

Allows the user to specify a mnemonic to generate 1-n exit messages depending on how many validator indices are provided. The success of this command will result in n signed_exit_transaction-***.json files in exit_transactions for each validator index provided.

Testing

Build as instructed

Create a new mnemonic and keystore(s) on mainnet with
./deposit.sh new-mnemonic

For each keystore, create an entry in this base offline-preparation.json file:

{
  "version":"3",
  "validators":[
    {
      "index": "<ANY_INDEX_IE_123456>",
      "pubkey": "0x<PUBKEY>",
      "state": "active_ongoing",
      "withdrawal_credentials": "<CREDENTIALS>"
    }
  ],
  "genesis_validators_root":"0x4b363db94e286120d76eb905340fdd4e54bfe9f06bf33ff6cf5ad27f511bfe95",
  "epoch":"274294",
  "genesis_fork_version":"0x00000000",
  "exit_fork_version":"0x03000000",
  "current_fork_version":"0x04000000",
  "bls_to_execution_change_domain_type":"0x0a000000",
  "voluntary_exit_domain_type":"0x04000000"
}

With the saved offline-preparation.json above in the same location as ethdo, create an exit message with ethdo using the following command:
./ethdo validator exit --offline --mnemonic="<GENERATED_MNEMONIC>" --validator=<VALIDATOR_INDEX_IN_PREF_FILE>

Create an exit message with the CLI using both
./deposit.sh exit-transaction-mnemonic --epoch=274294
for each keystore
./deposit.sh exit-transaction-keystore --epoch=274294

All outputs should match