KIP-483 — Add Broker Version API in Admin Client

Discussion AdminProtocol

Proposes adding an `AdminClient.describeCluster()` extension or a dedicated API to expose the Kafka broker version (not just API version ranges) to client applications. While `ApiVersionsResponse` lists supported API version ranges, clients that need to make decisions based on the exact broker version (e.g., feature flag checks as in KIP-447) have no direct way to retrieve it.

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