praefect: Check of the service readiness with RPC call

For the praefect binary we have a sub-command to verify if praefect service can operate without issues. The verification process checks if migrations were applied, if gitaly nodes are reachable, if the clock synced, etc. This check can be done only when you have direct access to the binary. With introducing of ReadinessCheck RPC we now can run the same verification process mentioned above by issuing an RPC call. The new RPC will be part of the gitlab:gitaly:check task. It is noop for the gitaly service as of now.

Part of: gitlab#348174 (closed)

Once we release this change the next step would be to include invocation of it in the gitlab:gitaly:check task.