KIP-619 — Add internal topic creation support

Discussion ProtocolBroker

Introduces a mechanism for Kafka clients to register their own internal topics with the broker via a new `RegisterClientMetrics` RPC so that the broker's `MetadataResponse` can correctly mark those topics as internal. Broker-side internal topic detection relied on a hardcoded list (`__consumer_offsets`, `__transaction_state`), so client-defined internal topics like Connect's `connect-configs` were not recognized as internal by the broker and could be incorrectly exposed.

Protocol Impact

Metadata · UpdateMetadata

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