Allow stateful migration from StreamingFileSink to FileSink

Flink-1.12 introduces the `FileSink` in FLINK-19510, which can guarantee the exactly once semantics both in the streaming and batch execution mode. We need to provide a way for the user who uses `StreamingFileSink` to migrate from `StreamingFileSink` to `FileSink`.

For this purpose we propose to let the new sink writer operator could load the previous StreamingFileSink's state and then the `SinkWriter` could have the opertunity to handle the old state.