AC Flows -> New action: Wait for callback

Given that now we can call javascript it would be great to have an action in flows that just waits for a callback to continue with the next steps.

So basically the async/promise and await workflow.

Right now if we call a javascript there is no easy way to return to the flow that initiated the call.

I am thinking in the terms of adding the wait for callback step and getting automatically a piece of code to copy and paste in our callback function et voila the flow continues where it was waiting.