[oracle] Support specific-offset startup mode with SCN for Oracle pipeline source

[fightBoxing]

Motivation

The Oracle pipeline connector currently only supports initial / snapshot / latest-offset startup modes, while the MySQL pipeline connector already supports specific-offset. For Oracle users who want to skip the full snapshot phase and start incremental (LogMiner redo log) consumption from a known position, there is no way to specify a starting SCN.

Note: the underlying Debezium property log.mining.start.position.scn cannot be used as a workaround, because the shaded Debezium dependency in the connector jar does not include that configuration, and passing it through the debezium.* prefix is silently ignored.

Modification

• OracleDataSourceOptions: add new option scan.startup.specific-offset.scn (Long, no default value)
• OracleDataSourceFactory: add specific-offset branch in getStartupOptions(), which builds StartupOptions.specificOffset(Map{scn, commit_scn}). The base framework (StreamSplitAssigner#createStreamSplit) already natively consumes SPECIFIC_OFFSETS offsets, so no framework change is needed.

Usage

source:
  type: oracle
  scan.startup.mode: specific-offset
  scan.startup.specific-offset.scn: 2053676

Verification

Verified end-to-end on a real cluster (Flink 1.20.1 YARN session + Oracle 12c LogMiner + Iceberg sink):

• JobManager log confirms the stream split starts exactly at the specified SCN: Assign split StreamSplit{offset={commit_scn=0, scn=2053676}}
• Only changes committed after the specified SCN are captured; all pre-existing snapshot rows are correctly skipped