[services] Export comm's grpc generated files as cmake project

Summary:
Allows for packaging of the grpc generated files, this allows for
downstream packages such as tunnelbroker to then find_package(comm-grpc)
and use the related targets and avoids needing to interact with
code generation logic.

Depends on D4294

Test Plan:
nix build .#comm-grpc

Then inspect ./result

Reviewers: max, karol, varun, ashoat

Reviewed By: max, karol, varun, ashoat

Subscribers: varun, ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D3867