[ML-76] Reorganize multiple Spark version support code structure

[jerrychenhf]

What changes were proposed in this pull request?

1. Standardize the profile based source code organization as following convention:

project-root
--src
----main -> common code entry
----test -> common test code entry
--spark-3.0.1 -> version specific entry
----main -> version specific code entry
----test -> version specific test code entry
--spark-3.1.1
----main
----test
...

2. Use build-helper-maven-plugin to add additional version specific source code to build process.
3. Additionally, as part of this refactor, change maven-scala-plugin (old) to scala-maven-plugin (new)

[github-actions]

#76