Implement Reportable protocol for Oban

[unflxw]

Spun off from #996.

Oban's documentation provides a guide on how to implement a Reportable protocol to provide custom logic on whether a given error should be reported. This allows for a superset of the functionality provided by report_oban_errors.

Note that Oban does not actually implement any Reportable protocol; its documentation guide merely suggests a pattern to implement in your application and tie to your own telemetry handlers.

As providers of telemetry for Oban, we could implement some sort of Appsignal.Oban.Reportable protocol ourselves to provide this sort of functionality. This would save customers from having to implement this functionality and attach it to AppSignal, effectively re-implementing our Oban instrumentation.

(Open question: how should this interact with the report_oban_errors config option? My gut feeling is the reportable behaviour should override it)