feat: optimize Oracle mode write with multi-row MERGE

[yuanoOo]

Summary

• Add Oracle multi-row upsert SQL generation by extending the dialect with a row-count aware getUpsertStatement implementation.
• Route Oracle keyed upsert flushing to a multi-row MERGE ... USING (SELECT ... UNION ALL ...) path, while preserving existing insert/delete behavior.
• Refine default JDBC properties by enabling MySQL batch rewrite options only in MySQL mode and add tests for Oracle dialect SQL generation and end-to-end multi-row merge behavior.

Test plan

• mvn -pl flink-connector-oceanbase -Dtest=OceanBaseOracleDialectTest test
• OceanBaseOracleConnectorITCase#testSink (run with Oracle env)
• OceanBaseOracleConnectorITCase#testMultipleTableSink (run with Oracle env)
• OceanBaseOracleConnectorITCase#testMultiRowMergeUpsert (run with Oracle env)