feat: support broker node eligibility filter in TieredBrokerSelectorStrategy

[jtuglu1]

Description

Adds a hook for router routing strategies to hook into which set of brokers are actually valid to route to. Useful for filtering based on other attributes other than service name, for example maintaining version=2 routers only query version=2 brokers.

Release note

Support broker node eligibility filter in TieredBrokerSelectorStrategy.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[FrankChen021]

I have reviewed the code for correctness, edge cases, concurrency, and integration risks; no issues found.

Reviewed 3 of 3 changed files.

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This is an automated review by Codex GPT-5.5