[Feature] RFS support for external versioning

[kanatti]

We have some use cases where the version number (version) of documents is set externally—typically stored as a field in the _source (e.g., @version_number). Currently, RFS does not support setting external versions when making _bulk calls.

It would be useful to allow RFS to:

• Extract the version from the source(maybe using transformations)
• Set version_type as external
• Optionally ignore version conflicts and continue processing instead of failing the worker

This would give us more control over versioning behavior during reindexing and unlock support for externally versioned documents.