[SPARK-17025][ML][PYTHON] Persistence for Pipelines with Python-only Stages

## What changes were proposed in this pull request?

Implemented a Python-only persistence framework for pipelines containing stages that cannot be saved using Java.

## How was this patch tested?

Created a custom Python-only UnaryTransformer, included it in a Pipeline, and saved/loaded the pipeline. The loaded pipeline was compared against the original using _compare_pipelines() in tests.py.

Author: Ajay Saini <ajays725@gmail.com>

Closes #18888 from ajaysaini725/PythonPipelines.