KubernetesPodOperator new callbacks and allow multiple callbacks

[johnhoran]

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I would like to have multiple callbacks in the kubernetes pod operator, and add two new callbacks. on_manifest_finalization would allow the callback to make changes just before the manifest is turned into a pod.
on_pod_wrapup would happen after the calls to on_pod_completion but just before the pod is deleted.

Adding both of these plus allowing multiple callbacks would allow you to do things in the kubernetes pod operator akin to how it approaches XComs but in a modular way. My use case here is I'm running DBT in kpo, and I want to do multiple things to the DBT artefacts after the DBT job has run, I could use on_manifest_finalization to insert an alpine sidecar with volumes mounted with the same intention as the XCom sidecar, where the sidecar keeps the volumes alive as files are extracted from it. on_pod_wrapup would allow me to insert a single sidecar and have multiple callbacks run their on_pod_completion before on_pod_wrapup kills the sidecar.