Streamline serialization when retrieving spatial data through fields. #60259
Description
In #59821 we added support for retrieving spatial values like geo points using the fields parameter. To keep the PR well-scoped, we adopted a simple but inefficient serialization strategy: parsed geometry object -> xContent -> simple Java object.
We plan to streamline the serialization by converting directly from geometry object -> simple Java object. It is important to avoid unnecessary conversions, as some geometry objects can be quite large (like polygons representing a complex boundary).
Relates to #55363.