[services][feature-flags] Introduce features endpoint

Summary:
Introduce a new endpoint /features that handles GET requests with query params containing platform, code_version and staff flag, e.g. /features?code_version=130&is_staff=false&platform=iOS.

Depends on D6860

Test Plan: Send a request to an endpoint and check if provided query params can be accessed.

Reviewers: michal, bartek, jon, varun

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6861