KIP-234 — add support for getting topic defaults from AdminClient

Discussion Admin

Adds a `describeClusterDefaultConfigs()` method to `AdminClient` that returns the broker-level default values for all topic configs without requiring an existing topic. Users creating topics via the AdminClient cannot determine broker default values ahead of time, leading to topics being created with incorrect configs (e.g., `delete` cleanup policy instead of `compact`) that must be fixed post-creation.

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