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Compatibility, Deprecation, and Migration Plan

No compatibility issues are anticipated.   SpecificallyRegarding backwards compatibility:

1. The interface methods have a default implementation.
2. The proposed method name (writeWatermark) is new to the Flink APIs to further minimize the impact. avoid possible collisions.

Regarding binary compatibility of API signatures (japicmp):

1. SinkFunction is already excluded from the checks that are performed.
2. SinkWriter is an Experimental interface (as of Flink 1.13), all of which are excluded from the checks that are performed.

Test Plan

The Flink codebase has an integration test (FileReadingWatermarkITCase) which involves counting watermarks that arrive at a test sink.   We will update the test code to use the new functionality.

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