[services][feature-flags] Add configuration for http server

Summary:
The only thing we configure at this point is the port. Later we're going to also configure https and related keys.

Depends on D6858

Test Plan: Tested with the rest of the stack.

Reviewers: michal, bartek, jon, varun

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6859