Add CredentialRepositoryV2 to supersede CredentialRepository

[iaik-jheher]

This is part of the UserIdentity work I proposed in #289 (comment); I am breaking it down into smaller parts for ease of review.

This part supersedes the current CredentialRepository interface with a new CredentialRepositoryV2 interface. CredentialRepositoryV2 works based on UserIdentity objects, not usernames.

The CredentialRepository interface extends CredentialRepositoryV2, and has appropriate default implementations to allow existing repositories to continue to function unmodified.

(This pull request does not actually use the V2 interface or its functions anywhere, yet.)

[emlun]

This is now superseded by experimental release 2.6.0-alpha4, which includes a new CredentialRepositoryV2 aiming to solve the same problems, but with a different architecture. I hope this supports your use case; if not, please let us know!