Revival of GH-7371: Added vector union support for Python

[jtdavis777]

I wanted to revisit GH-7371 (fixes #4530) to add Advanced Unions support to python.

All credit for the base work goes to @surculus12 (and I cherry picked their commits over to preserve authorship) - I'm only coming in to get it up to date with the current repo and hopefully get it across the finish line.

I will address the last PR comments left on the original PR as and if they still apply.

[jtdavis777]

One thing I either don't understand or want to fix, is how to get a member of a vector of union back to its 1st class type. right now the function returns a Table type with no obvious way to get back to the union type. May just be my inexperience with the generated python code. looking at the python examples in the documentation, this is normal.

[jtdavis777]

@fliiiix would you be up for helping me polish this PR up? I just rebased the original commits onto (a more recent) master, but enough things changed I would appreciate a second set of eyes to point out what still needs work.