Add RPCs to access GroupContextExtensions and maybe roster

[bifurcation]

As @mulmarta points out in #151, it would be good to be able to examine the GroupContextExtensions that a client has associated to a group. This would let us confirm that GCE proposals and branch/reinit extension changes have been applied correctly, and might simplify management of external signers.

It might also be useful to be able to access the roster, specifically the list of basic identities in non-blank leaves in left-to-right tree order. This would allow the test runner to explicitly confirm that adds / joins had succeeded.