This change introduces optional source filtering directly within source loaders (both synthetic and stored).
The main benefit is seen in synthetic source loaders, as synthetic fields are stored independently.
By filtering while loading the synthetic source, generating the source becomes linear in the number of fields that match the filter.

This update also modifies the get document API to apply source filters earlier—directly through the source loader.
The search API, however, is not affected in this change, since the loaded source is still used by other features (e.g., highlighting, fields, nested hits),
and source filtering is always applied as the final step.
A follow-up will be required to ensure careful handling of all search-related scenarios.