NiFi 2.x support

[soenkeliebau]

There are a couple of new features in NiFi 2.0 that would make sense to natively support in our operator.

Specific items are tracked as sub-issues of this epic.

Also as we can not add PRs as sub-issues:

• docs: Add docs on how use custom Python processors #753 => See https://docs.stackable.tech/home/stable/nifi/usage_guide/custom-components/custom-python-processors/