[services][feature-flags] Add hello world http server
Just a hello world http server. There are a couple of interesting servers available in Rust, but I've chosen this because it is relatively high level, performant, and easily integrates with https.
Depends on D6859
Test Plan: Send GET to / and check if response is returned. Checked if both 127.0.0.1 and [::1] work.
Reviewers: michal, bartek, jon, varun
Reviewed By: bartek
Subscribers: ashoat, atul
Differential Revision: https://phab.comm.dev/D6860