feat(pyspark): add mode computation

[KDra]

Is your feature request related to a problem?

No

What is the motivation behind your request?

table.describe() is a very useful feature that I would like to also run on PySpark tables

Describe the solution you'd like

I could implement it myself but I am unsure where to start

What version of ibis are you running?

10.5.0

What backend(s) are you using, if any?

PySpark

Code of Conduct

• [x] I agree to follow this project's Code of Conduct

[cpcloud]

You can add ops.Mode: "mode" to the SIMPLE_OPS mapping in the PySpark compiler, then run the test suite with pytest -m pyspark -k mode and remove the "pyspark" string from whichever tests are now passing.