[INLONG-3791][Sort] Add WatermarkField definition to support transform

[yunqingmoswu]

Title Name: [INLONG-3791][Sort] Add WatermarkField definition to support transform

Fixes #3791

Motivation

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Modifications

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Verifying this change

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• This change is a trivial rework/code cleanup without any test coverage.

• This change is already covered by existing tests, such as:
  (please describe tests)

• This change added tests and can be verified as follows:

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  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

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