[identity] Move proto generated modules to one place

Summary:
In D9957 I added a comment about problems with include_proto modules. This diff addresses that by moving all proto includes to one place.

Depends on D10051

Test Plan:
Git grepped all references to client, unauthenticated etc to make sure I've changed them all.
Identity service builds and starts.

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10052