Implement framework for flexible 2FA

[BryanJacobs]

This adds support for using the hmac-secret FIDO extension to contribute keying material for a KeePass 4 file.

It does this by storing an additional XML statekeeping blob in the outer ("public") header. This blob is designed to hold a variety of different authentication factors, such as passwords, key files, and Yubikey challenge-response devices.

This is a reopening of the accidentally-closed #373 . Sorry about that.

[mathben]

PR about ticket #311