CREDENTIAL: Allow protocol-based registration.

As discussed in the thread around [1], this patch adds a mechanism by
which an origin may register itelf as supporting a particular protocol,
and by which that registration may be used to populate a chooser when no
credentials are directly available for a relying party.

[1]: https://lists.w3.org/Archives/Public/public-webappsec/2015Sep/0018.html

Author: mikewest