KIP-893 — The Kafka protocol should support nullable structs

Accepted Kafka 3.5 Protocol

Adds nullable struct support to the Kafka RPC protocol schema, enabling struct fields to be null on the wire (serialized as INT8 -1/VARINT 0 presence byte before the struct bytes). This is needed for KIP-848's ConsumerGroupHeartbeat response, which must be able to return null assignment to indicate no change, rather than re-sending the full assignment every heartbeat.

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