[terraform] set up keyserver load balancer

Summary:
This implements the load balancer for the keyserver ecs cluster, distributing keyserver requests to keyserver tasks

Depends on D12559

Test Plan:
Successful load balancer health check on the primary ecs task, meaning traffic is properly sent to our fargate tasks.

Was also able to send requests through my domain configured to point to the load balancer.

Reviewers: bartek, kamil

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12560