[FEATURE] upgrade Spark DataSource V2 APIs for unify streaming and batch

[jiazhai]

Thanks to @hanguyen6's proposal. in PR #42

Spark DataSource V2 APIs has been refactored to unify between streaming and batch. Should we upgrade our code to follow the updated abstraction:

https://issues.apache.org/jira/browse/SPARK-25390

write API refactor: https://docs.google.com/document/d/1vI26UEuDpVuOjWw4WPoH2T6y8WAekwtI7qoowhOFnI4/edit#

read API refactoring: https://docs.google.com/document/d/1uUmKCpWLdh9vHxP7AWJ9EgbwB_U6T3EJYNjhISGmiQg/edit#

I have updated code to follow that abstraction and can help if needed.

[jiazhai]

@hanguyen6 would you like to work on this issue?

[hanguyen6]

@jiazhai Yes, I am working on it right now. Will update when it completes

[hanguyen6]

@jiazhai, @jianyun8023 Could you pls review PR #50 ?