Upgrade XmssKeyManager utility

[anshalshukla]

Currently the XmssKeyManager isn't completely configurable. Let's make it more generic, few pointer to include (non exhaustive list):

• create_and_store_key_pair offer very little control to user. Add activation_epoch, active_epochs, seed, etc to give complete control over key generation
• save the private keys along with test vectors to help client teams verify results
• we are padding the signature_bytes before writing them to Signature object, this look kinda ugly, see if there is a way this can be better handled (sign_attestation)
• anything else to make it generic and give users complete control over key generation

[vyakart]

I'd like to work on this:),I'll open up a PR