Add DataFrame fill_null

[kosiew]

Is your feature request related to a problem or challenge?

There is a common operation in libraries such as pyspark to fill nulls in an entire DataFrame (or to limit by columns). It would be nice to have a similar feature in datafusion and datafusion-python.

Describe the solution you'd like

If I have a dataframe with a bunch of null values in different columns, I would want to replace all nulls in those columns with the provided value IF it can be cast to the column's type. Otherwise no-op should happen. Also the user should be able to limit which columns this applies to.

Describe alternatives you've considered

Instead of having a built‑in fill_null, you can use conditional expressions or functions like coalesce (or nvl) to replace nulls or NaNs.

Additional context

This is a repost from apache/datafusion-python#922, prompted by this PR comment