[translator] Support per-field (selective) translation within a run

[denis-urban]

Summary

Translation operates on the whole document — every explicitly-localized leaf field is translated. Field exclusion (withFieldTranslation({ exclude: true })) is static schema config. Add the ability to translate only a selected subset of fields in a given run.

Current behavior

• The pipeline walks and translates every explicitly-localized leaf field of the document.
• The only way to skip a field is the static custom.translateKit exclusion in the collection config; there is no per-run choice.

Proposed change

• Accept an optional field-path include/exclude set on the enqueue input and thread it into the pipeline's field walk.
• Optionally expose a field picker in the document panel so an editor can translate just the fields they changed.

Acceptance criteria

• enqueue accepts an optional field selection (include or exclude list of field paths).
• The pipeline sends only the selected fields to the provider; unselected localized fields are untouched.
• Backward compatible: no selection ⇒ translate all (current behavior).

Notes

Listed under the plugin roadmap ("Field-level translation"). Static withFieldTranslation exclusion remains as-is and composes with the per-run selection.