Construct Uri from Enrolled method

[DanGould]

Enrolled should have all of the configuration and state necessary to return a Uri. This should enable removal of the less clear pubkey, fallback_target functions.

This allows construct_pj_uri can to be removed from the v2 cli and any similar function from downstream implementation.

Manual construction is particularly annoying because OhttpKeys is required for the enroller and then again for the Uri, and it'd have to be cloned in the Uri without this convenience function.