Move TextExtractor service to AI core

Problem/Motivation

Follow-up to ✨ Attempt to find nested/referenced text field values for content suggestions Active : AI content suggestion and AI translate (probably automators as well) have very similar requirement to extract and then insert textual metadata from/into content entities.

Such a service exists in ai_translate namespace (TextTranslator service + FieldTextExtractorInterface plugins), but it should be moved into core to be available to other submodules and contrib