add POWA support

[david-ecc]

Please add support for POWA ("PostreSQL Workload Analyzer"): https://powa.readthedocs.io/en/latest/

[FxKu]

There's already a long discussion thread about supporting monitoring and we repeatedly stated, that we regard this out of scope for the operator. You will see from the thread that people have started using different solutions by now that work best for them.

Still, our community could check what could be done in the operator, to make it easier for external tools to hook in. In the case of PoWA (great project btw.), one ToDo could be to add extensions it relies on to the Spilo image, for example.