Streamline serialization when retrieving spatial data through `fields`.

[jtibshirani]

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.

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Geo)

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)