[Bug][Flink] Move Flink-specific flush coordination from API to translation layer

[CloverDew]

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• I had searched in the issues and found no similar issues.

What happened

Problem
After the merge of #9867, the SupportSchemaEvolutionSinkWriter interface contains Flink-specific methods (e.g., handleFlushEvent, getSchemaCoordinator). The implementation details of a particular engine should not be introduced into the general API.

Solution
Refactor to decouple the API:

1. Keep only generic methods (like applySchemaChange) in the SupportSchemaEvolutionSinkWriter interface.
2. Move all Flink-specific logic (flush coordination, SchemaCoordinator interaction) into the FlinkSinkWriter implementation.

Related PR

• Introduced by: [Feature][Flink] Support CDC source schema evolution on Flink engine #9867

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Zeta or Flink or Spark Version

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Are you willing to submit PR?

• Yes I am willing to submit a PR!

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