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