[feature](coordinator) Add backend selection extension framework (#65173)

[deardeng]

pick from #65173

This PR introduces a public backend selection extension framework for query, load, and replica-repair scheduling paths. The public interfaces are aligned with the downstream implementation so that downstream extensions can be maintained without carrying kernel API differences. Apache Doris does not provide or register an affinity BackendSelectionProvider. The default community provider is strictly no-op, preserving existing query scheduling, load candidate ordering, placement, and replica-repair source selection behavior. This PR does not introduce resource-group affinity implementation, enterprise provider, enterprise configuration, or other enterprise-only behavior.

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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4. Which code was refactored and why was this part of the code refactored?
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[deardeng]

run buildall

[hello-stephen]

Cloud UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	78.50% (1858/2367)
Line Coverage	65.27% (33059/50650)
Region Coverage	52.30% (31170/59593)
Branch Coverage	56.24% (9386/16689)