[services][feature-flags] Query the db

Summary:
The final diff about accessing the database.

Depends on D6856

Test Plan: Create some dummy data in the db. Query the db, log the result and verify that the parsed data is correct.

Reviewers: michal, bartek, jon, varun

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6857