[keyserver] add health check route to keyserver for use by load balancer

Summary:
We require this route for load balancer health check

Depends on D12531

Test Plan: Later in stack, usage of load balancer results in healthy ecs tasks

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12558