[native] expose module with generated tonic client code

Summary: the build.rs script generates rust code from the identity.proto file. we then use the include_proto macro to expose the generated code as a module in our library

Test Plan: cargo build

Reviewers: tomek, karol, jon

Reviewed By: tomek

Subscribers: jon, ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4706