generate-key should support an implicit account creation

[ilblackdragon]

We are adding implicit account creation, where account_id = hex(public_key).
Hence, there should be a way to create such account via generate-key.

This should create new key pair, and based on it, create the file in credentials with account id that matches the encoded key.

Additionally, people may want to use custom mnemonic phrase for their keys. E.g. there should be a way to near generate-key --mnemonic="tree forest ... bark". This way instead of generating new key pair, it will use this mnemonic (and possibly additional HD path) to get public key.