Better typing for pyspark backend

[grihabor]

Description of changes

I've been running mypy and changing a bunch of type definitions for pyspark backend to be fully typed. Now mypy has no errors in the pyspark backend module if you run it like this:

uv venv
uv pip install .[pyspark,polars] mypy pytest pandas-stubs pyarrow-stubs
uv run mypy ibis/backends/pyspark/

Some of the changes are very opinionated and are open for discussion

[cpcloud]

Not opposed to this but can you make only the typing changes and move variable renaming to a different PR?

[grihabor]

and move variable renaming to a different PR

Unfortunately, mypy doesn't allow reassigning variables with values of a different type. So using the same variable names is actually breaking type checking

[grihabor]

I could do it in separate pr, but that would be just more confusing. I think the intention is more clear when everything related to typing is in one pr