## What changes were proposed in this pull request?

When we pass a Python function to JVM side, we also need to send its context, e.g. `envVars`, `pythonIncludes`, `pythonExec`, etc. However, it's annoying to pass around so many parameters at many places. This PR abstract python function along with its context, to simplify some pyspark code and make the logic more clear.

## How was the this patch tested?

by existing unit tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11342 from cloud-fan/python-clean.