Webhook support

[matvp91]

Is your feature request related to a problem? Please describe.
Today when you want to figure out if a job finished, you'll have to manually poll the api.

Describe the solution you'd like
Instead of periodically asking whether a job is finished, allow a job to include a list of "webhook" URL's that the job can fetch when finished.

Additional context
Sieve (https://docs.sievedata.com/guide/custom-apps/using-webhooks) does it nicely, they allow you to specify a couple of URL's with their corresponding webhook type. This way the entire job processing is stateless and does not require a DB to store the webhook URL.