[subtask][spark]Design docs for support batch insert/update/delete/merge-into

[baiyangtx]

This is a sub-issue, parent task is #173

some questions should be clear at design docs. i. pos-delete or eq-delete for unkeyed table? ii. should batch update write log store? iii. insert command write to base store or change store? iv. should we keep hive compatibility visible order for writed snapshot(with flink write)