[feat][broker][branch-3.3] PIP-321 Introduce allowed-cluster at the namespace level (#22378)

[Demogorgon314]

(cherry-picked from commit 36bae69)

Motivation

Cherry-picked from #22378

Modifications

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This change is a trivial rework / code cleanup without any test coverage.

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

Does this pull request potentially affect one of the following parts:

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• Dependencies (add or upgrade a dependency)
• The public API
• The schema
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• The threading model
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• The admin CLI options
• The metrics
• Anything that affects deployment

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