[KYUUBI #7379][1/4] Data Agent Engine: module skeleton, configuration, and engine core

[wangzhigang1999]

Summary

Part 1 of 4 for the Data Agent Engine (umbrella issue, KPIP-7373).

New module externals/kyuubi-data-agent-engine with the engine fully runnable via Echo provider.

• Module pom.xml, build/dist packaging, all kyuubi.engine.data.agent.* configuration entries
• DATA_AGENT engine type, EngineRef branch, DataAgentProcessBuilder with API key redaction
• Engine core: DataAgentEngine, BackendService, TBinaryFrontendService, SessionManager/Impl, OperationManager
• DataAgentOperation, ExecuteStatement with async agent execution and event-to-JSON conversion
• IncrementalFetchIterator for thread-safe producer/consumer streaming
• Event system: AgentEvent, EventType, all lifecycle/content/tool events
• EchoProvider for testing

Test plan

• IncrementalFetchIteratorSuite — basic ops, position tracking, concurrent safety
• IncrementalFetchIteratorCompactionSuite — compaction behavior
• DataAgentOperationSuite — JDBC round-trip with Echo provider
• EchoProviderTest — event sequence and content echo
• EventTest — immutability, truncation, null handling, SSE names
• DataAgentProcessBuilderSuite — main class, API key redaction, memory, classpath