Full OAuth2 Support

[lilyball]

We now have the HTTPAuth infrastructure so people can roll their own. But it would be nice if we had an already-written full implementation of OAuth2, including bearer tokens, so people can just use it without writing their own auth implementation.

Ideally we'd do this in a way that the access token type is generic, and we'd provide the implementation of bearer token. This way anyone who needs a different access token type can still use the overall OAuth2 support and just implement the access token part themselves.

[ekimia]

This could be a bit of a rabbit hole given all the different implementations of OAuth 2. However there are other things to do to make it easier to hook up. An info object could be made that includes an authorizationURL along with type to bring up a safari view controller for auth given this a common use case of OAuth 2. If that bit is done, OAuth 2 support for specific providers would be neat to have in another framework.