[KIP-664] Implement transaction management Admin APIs

[Mario Longobardi (mlongob)]

Add four new Admin APIs for detecting and aborting hanging transactions as specified in KIP-664:

• ListTransactions: Lists active transactions across all brokers with filtering by state, producer ID, duration, and transactional ID pattern. Supports API versions 0-2 with automatic version negotiation.

• DescribeTransactions: Describes detailed transaction state including topics/partitions involved, producer epoch, and timing information. Routes requests to the appropriate transaction coordinators.

• DescribeProducers: Retrieves active producer state from partition leaders including producer ID, epoch, sequence numbers, and transaction offsets.

• AbortTransaction: Aborts hanging transactions by writing abort markers via the WriteTxnMarkers API. Takes producer state from DescribeProducers to construct the abort request.

All APIs include:

• Public C API with opaque types and accessor functions
• Kafka protocol request/response serialization
• Admin fanout for multi-broker coordination
• Mock cluster handler for ListTransactions
• Comprehensive tests with mock and real cluster coverage
• Example programs demonstrating usage

[confluent-cla-assistant]

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