Add presentation message

[christianpaquin]

Add a presentation challenge to the proof transcript to encode a verifier challenge for interactive presentations, or a timestamp+verifier UID for non-interactive ones. This could also encode payload to be signed by the user's credentials. See section 3.3 of the Crescent paper for details.

This will likely be split in multiple PRs:

• update library to support accept arbitrary bytes
• add higher level API to support various use cases
• add functionality to the sample