[Identity] Reference new proto location

Summary:
Update identity reference to new protobuf location.
Also update dockerfile to have a similar structure to the repo to avoid
issues around symlinking or other hacks to make the file present.

Test Plan:

# Outside docker
nix develop # or install rust and protobuf 3.15 + grpc
cd services/identity
cargo test

# Docker
docker build . -f services/identity/Dockerfile

Reviewers: varun, atul, abosh, tomek

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5125