refactor(iceberg): move scan from batch to connector

[xxchan]

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What's changed and what's your intention?

refactor(iceberg): move scan from batch to connector

Add function , so the code can be shared by stream source.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note

[xxchan]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: support streaming iceberg source #20568
• refactor(iceberg): move scan from batch to connector #20567 👈 (View in Graphite)
• refactor: remove unnessary check_iceberg_source #20566
• ci: merge label for iceberg tests & add auto labeler #20574
• main

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