Fix HoodieIncr source checkpoint not progressing and add support from drop / cast columns

If there is no new commits to process in current batch , checkpoint value that is being returned is wrong .
Adding support to drop columns 

Adding support to cast columns  from [array , struct ] type to str

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-3447
- Type: New Feature

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## Comments

29/Mar/22 17:48;xushiyan;[~harsh1231] looks like there are 2 separate tasks in this ticket. can we split them?;;;

Start at the HoodieIncr source checkpoint handling and the column-processing entry points. First separate the checkpoint fix from the column changes, then trace how empty batches and drop/cast requests are represented. Done means the checkpoint is correct when no commits are available and array or struct columns can be dropped or cast to strings, with tests covering each behavior.

Written by the indexing model from the issue text.