Support custom NiFi components via git-sync

[soenkeliebau]

We already have docs how to use Python processors in https://docs.stackable.tech/home/stable/nifi/usage_guide/custom-components/custom-python-processors/.
This issue is about implementing a CRD change, which offers this functionality more conveniently.

• support functionality similar to AirFlow for running gitsync to get python processors into the running container
  • Move gitSync struct from airflow-op into operator-rs!
  • also support extra volumes for this
• supply Python runtime in container
• investigate if we want to offer separate functionality for dependencies to what NiFi does (especially considering airgapped environments)

Tasks:

• Support custom NiFi components via git-sync #793
• stackabletech/decisions#52
• stackabletech/decisions#53
• feat(stackable-operator): Add git-sync support operator-rs#1024
• feat(nifi): Add git-sync docker-images#1107
• Move the git-sync implementation to operator-rs airflow-operator#623