KIP-915 — Txn and Group Coordinator Downgrade Foundation

Accepted Kafka 3.5 TransactionsConsumer

Introduces versioned record types and downgrade support for the transaction coordinator (`__transaction_state`) and group coordinator (`__consumer_offsets`) by adding version headers and unknown-record skipping logic. Without this foundation, adding any new record field or type to these internal topics permanently prevents downgrading to an older broker version.

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