Extract Generators and Catalyst Python Improvements

Refer to design discussion here: https://discourse.paraview.org/t/extracts-exports-and-catalyst-python-scripts/4569

A few outstanding issues/features to be addressed subsequently:

• #20018 (closed)
• #20020
• #20021
• #20022
• #20023 (closed)
• #20032 (closed)

Users guide updates under development here: paraview-guide-ce!96 (merged)

Fixes #19931 (closed)