[terraform] initial ecs setup for keyserver primary

Summary:
Drafting this for some initial feedback. This sets up ecs cluster with primary keyserver task def

Depends on D12475

Test Plan: Successfully running the the keyserver task in a docker container

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12531