Separate the spark-commons module for Spark independence

## Background
Any library using `spark-commons` ver, 0.3 (or eventually higher), will need to decide if to work on Spark 2 or Spark 3. And that's despite majority of the code is independent of the Spark version used.

## Feature
Separate `spark-commons` to `spark-commons-core`, and `spark-commons` for version 2 and version 3. Where core is code that is Spark version ambiguous.