Support of credentials with specified security provider

This provides the ability to delegate authentication for a new user (not yet stored in the repository) to a specific security provider.
Which is especially useful (needed) for SSO integrations where only the (new) userId but not a password is available.

Login without a password is normally not allowed (enforced by the HippoLoginModule).
But for such SSO integrations this now will be allowed only if within the Credentials the "providedId" attribute is set, leaving it up to its corresponding security provider to verify/allow this.