[services][terraform] Add feature-flags service

Summary:
Resolves ENG-2819.
Terraform setup for feature-flags service using ECS Fargate.
Mostly copy-paste from the existing services - see diffs for blob and electron-update in this stack.

Depends on D8635

Test Plan: It's already live on production.

Reviewers: tomek, michal, jon, varun

Reviewed By: jon

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8636