[INLONG-8953][Sort] Fix Iceberg source defaults the StartSnapshot to the latest

[vernedeng]

Prepare a Pull Request

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• Title Example: [INLONG-XYZ][Component] Title of the pull request

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• Fixes [Bug][Sort] Iceberg source default the StartSnapshot to the latest #8953

Motivation

Iceberg source force defaults the StartSnapshot to the latest and cannot be reset.

Modifications

Offer the option to allow the user to assign the start snapshot strategy from the earliest.

Verifying this change

(Please pick either of the following options)

• 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:

  (example:)

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

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a follow-up issue for adding the documentation