[Identity] Allow for setting logging levels through env

Summary:
Improve logging, previously only INFO and above
could be emitted. Now you can set debug with RUST_LOG=debug

Depends on D8400

Test Plan:

cd services/identity
RUST_LOG=debug cargo run -- server

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8401