[FLINK-39842][oracle-cdc] Support timestamp startup by resolving SCN

[paulo-t]

This PR makes Oracle CDC timestamp startup usable by resolving the requested startup timestamp to an Oracle SCN and using that SCN as the actual stream offset.

Oracle streaming positions are represented by SCN. The common timestamp startup option therefore needs Oracle-specific conversion before the stream split can start from the requested point in time.

Changes

• Create Oracle timestamp startup offsets.
• Resolve timestamp startup offsets to Oracle SCN before loading stream offset state.
• Add Oracle connection helpers for current SCN and SCN-to-timestamp conversion.
• Add local Debezium Oracle overrides needed by this connector version.
• Add unit coverage for offset creation, timestamp-to-SCN resolution, source config propagation, and table factory startup mode handling.

Tests

• Not run locally: Maven currently gets stuck before the test phase while launching Java (java --enable-native-access=ALL-UNNAMED -version).