enable webhooks for local testing of serverless workers

[andreemic]

Is your feature request related to a problem? Please describe.
We usually run our Docker containers locally and test their integration with the rest of our system, which uses webhooks. Whenever we add a new output from a model, we want to test it with the webhooks, but this is not currently possible.

Describe the solution you'd like
When running

python your_handler.py --rp_serve_api

and then calling /run, the webhook passed as input should be called in the same way it would with a deployed serverless worker.

Describe alternatives you've considered
Logging the output of our model instead of webhooks, or implementing our own webhook within our container. That works but can lead to inconsistent behavior / duplication of code that could be handled by RunPod and is probably useful to many users.