Support listening to cloud events in knative

This is a feature request...

Looking at the recent knative integration (#119) - this works great for actions listening to HTTP requests as it proxies between the HTTP request and the expected Web Action parameters/response.

This work could be extended to support listening to Cloud Events on Knative. This would allow those actions to be connected to knative [event sources](https://knative.dev/docs/eventing/).

Digging around in how Cloud Event receivers work - they use an SDK which just listens on a HTTP port for the incoming events and decodes to an object, e.g.
https://github.com/knative/eventing-sources/blob/master/cmd/event_display/main.go

We can just import the CloudEvents JS SDK to handle the HTTP transport and proxy the events to the handler functions. https://github.com/cloudevents/sdk-javascript