[services] expose opaque types in library

Summary: We have to use these types on the client side so I'm exposing them in a library so they can be reused.

Test Plan: Was able to use these structs in the client crate

Reviewers: tomek, karol

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4783