Optimize block loader of semantic_text field for synthetic source #126434
Description
semantic_text field always uses fallback synthetic source (implementation of synthetic source that uses a special _ignored_source field to store raw field data). If such fields are expected to be used with synthetic source it would be beneficial to apply optimization similar to #126237 to it. This would allows ESQL to directly load values from a stored field without the need to synthesize the entire synthetic _source.