Ability to supply additional information during credential acquisition

I think this is the final known gap in our credential acquisition and validation API/SPI.

There are a couple of specifications that also allow for additional information to be used when obtaining a representation of a users credential, the most obvious being the session based variant of digest authentication where a nonce and cnonce are also incorporated.

A second variant with two different modes of operation would be the realm associated with the digest credential, currently we assume it is tightly associated with the storage representation of the credential but it could also be the case that the mech is requesting it for a specific realm.