CLI for validator initialization with credentials

[Arkenan]

At the bare minimum, there should be an option to start the node with a credential path, which is parsed, and then saved it in the credential storage, to let the node know that it's running in the name of a specific validator and it should perform its duties when the slot/epoch/sync committee period requires so.

[MegaRedHand]

Other clients recommend using https://github.com/ethereum/staking-deposit-cli for key generation