[Identity] Use clap for argument parsing

Summary:
Use clap to unify usage of command line arguments
and environment variables

https://linear.app/comm/issue/ENG-4674

Test Plan:

cd service/identity
cargo run -- server --help # now shows optional arguments
cargo run -- server # works as intended

Reviewers: varun, michal, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9058