feat: Webhooks

[gersona]

• fixes Web Hooks #7604

[gersona]

@nijel before continuing, I'd like to have your input on a few points:

icon

Is it beneficial to add a new icon just for the webhooks add-on ? If so, I thought of something like the following, but am not sure if there's a repository we can get them from
https://icons8.com/icons/set/webhook--nocorrect

delivery failures and retries

To simplify things, I used celery tasks for the message delivery , which handles retries on failure.
Does it seem OK to your or do we need a way to track delivery status for each webhook event (which might require having a new model).

Payload

I more or less replicated what was included in the Fedora messaging payload. Is there something special to be added or removed ?

Payload signature

the standard-webhooks guidelines encourage to sign the payload, do we also need to provide a way to verify that signature in the code (a verify method that users can use), or should it just be explained in the documentation ?

[nijel]

I think I've addressed most of the questions inline, the only remaining bit is signature. We need an implementation for verification anyway for testing this in the testsuite. Maybe there is some standard function we can use here instead of reimplementing it?