Closes #20003: Introduce mechanism to register callbacks for webhook context

[jeremystretch]

Closes: #20003

• Simplify the signature of enqueue_event() to pass the request object directly (instead of the user & request ID)
• Attach the request object to the event when it is enqueued for processing
• Alter the signature of process_event_rules() to take a request object rather than a request ID
• Alter the parameters recorded for send_webhook tasks
  • Replace model_name with object_type
  • Add request if present
• Introduce the process_event_rules() function for use by plugins
  • Add webhook_callbacks to the NetBox registry
• Extend send_webhook() to gather additional context data from registered webhook callbacks
• Update event tests

[jeremystretch]

One bit I'm still not sure about: Should we namespace the context data to avoid collisions among callbacks from different plugins?

[mrmrcoleman]

Capturing some points here (I would like to be corrected on anything that I have misunderstood)

• In the namespacing scenario one would reference variables like payload.context.pluginname.key
• In the non-namespacing scenario one would reference variables like payload.context.key

The risk is that two plugins define the same key. My instinct tells me to namespace it because even if the risk of collisions is small the user impact is, presulably, large e.g. I get very difficult to solve issues that require me to introspect how NetBox works to figure out

[jeremystretch]

I have two counterpoints to share:

1. Plugins can always voluntarily namespace keys by attaching an arbitrary prefix (e.g. my_plugin_foo).
2. There may be cases where namespacing is undesirable. For example, support a webhook receiver wants to check for an error key set by any plugin. It is much simpler to check for context.error than to iterate through all (or some) dictionaries under context to determine if any of them contain error.

That said, I have no strong opinion either way.

[arthanson]

Should this be documented, maybe in the webhooks docs: https://netboxlabs.com/docs/netbox/integrations/webhooks/

[arthanson]

Looks good and works, just wondering about documenting this.