## What changes were proposed in this pull request?

This PR proposes to add `columnSchema` in Python side too.

```python
>>> from pyspark.ml.image import ImageSchema
>>> ImageSchema.columnSchema.simpleString()
'struct<origin:string,height:int,width:int,nChannels:int,mode:int,data:binary>'
```

## How was this patch tested?

Manually tested and unittest was added in `python/pyspark/ml/tests.py`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #20475 from HyukjinKwon/SPARK-23256.