Add wait_for_completion parameters to /deployments/artifacts/generate endpoint

Acceptance criteria:

• New boolean parameter added to /deployments/artifacts/generate endpoint, wait_for_completion.
• Default is false.
• If true, endpoint becomes synchronous, and returns when the artifact has been generated. If it is false, it behaves as before and returns immediately.
• If errors happen during the artifact generation, this is expected to be reported back to the API caller via an HTTP response code (Probably 400 Bad Request is most appropriate).
• Internally, this is implemented by having the deployment service poll the workflows service.