oracledb_cdc: add source_ts_ms metadata

[josephwoodward]

This change adds the source_ts_ms field to a message's metadata which represents the timestamp of when Oracle wrote the change record into the redo log.

It also removes redundant json encoding tags as they're irrelevant.

[claude]

Commits

1. Commit ab7d945 (oracledb_cdc: propagate the source timestamp to consumers) mixes code and documentation changes. The .adoc file update at docs/modules/components/pages/inputs/oracledb_cdc.adoc should be in a separate commit from the code changes, per the commit policy: "In multi-commit PRs, documentation changes must be in a separate commit from code changes."

Review
Adds source_ts_ms metadata to oracledb_cdc messages with the redo log timestamp in millis since epoch. Implementation is clean: guarded by a zero-time check, consistent with existing metadata patterns, and covered by integration test assertions.

LGTM