Expose some of session module's storage as public, as well as the set_keys transaction (#4175)

* Expose some of session's storage as public, as well as set_keys

Seemingly there's no reason not to do this, as anyone can always do
it the "hard way" by constructing storage keys or extrinsics.

* Use trait to expose `is_registered` function

* Missed removing a pub keyword

* Move trait to support, add docstrings