[services][feature-flags] Describe feature flags table in terraform

Summary:
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