Custom webhook response content

Some webservices support outgoing webhooks that could be pointed to st2, however they expect certain fields to be contained in the response. Feature request is to enable some way for st2 to return custom JSON payload in response to webhooks. One possibility would be to mark (in the yaml) which action st2 should execute and take the stdout to use as the webhook response. This would also require blocking the 200/202 response until 'webhook response' Action has completed.

One such webservice is Amazon's Alexa (Echo). Alexa can call an st2 webhook when some pre-configured 'Intent' is called, however Alexa expects the webhook response to contain instructions on how to proceed with interacting with the end-user. (This response could come from STDOUT of a script that st2 runs).

The issue names no files, tests, or entry points; start by tracing the existing webhook request/response handling and action execution flow. Done would mean a configured webhook can return a custom JSON payload, including action output when selected, and waits for that response action to complete.