[Identity] Refactor access token verification to be more usable

Summary:
Verifying if an access token is valid will be a common
action with many of the API calls. We should make this as ergonomic
as possible.

Part of https://linear.app/comm/issue/ENG-3943

Test Plan:
Set up identity service and localstack

nix develop

(cd services/identity && RUST_LOG=debug cargo run -- server)
(cd services/commtest && cargo test --test identity_access_tokens_tests)

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8094