[services][feature-flags] Connect http service with database

Summary:
Query the db, transform the result and return it as a response from an endpoint.

Depends on D6861

Test Plan: Send GET request and check it the response is correctly read from AWS.

Reviewers: michal, bartek, jon, varun

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6862