fix: make BrokerDynamicConfig backwards-compatible

[jtuglu1]

Description

Master tests are breaking. Experimental branch to fix them

• Rolling-forward deployment (HEAD broker -> v31 coordinator): receives empty broker dynamic config (same as if DNE)
• Rolling-backward deployment (v31 broker -> HEAD coordinator): receives empty broker dynamic config (old broker is unaware of new API)

Release note

Make BrokerDynamicConfig backwards-compatible. By default, brokers who poll coordinators without the broker dynamic config endpoint (and receive a 404) will return an empty BrokerDynamicConfig.

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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.

[kfaraz]

Thanks for the prompt fix, @jtuglu1 !