[services][feature-flags] Describe feature flags table in terraform
Add terraform description of the tables with feature flags configuration.
Depends on D6852
Test Plan: Tested if tables were successfully created on localstack. I'll also need to test it on AWS, but I haven't done that yet.
Reviewers: michal, bartek, jon, varun
Reviewed By: bartek
Subscribers: ashoat, atul
Differential Revision: https://phab.comm.dev/D6853