MAINT: Refactor ObjectDataProvider base class

This PR tries to add some firmer standardization to how metadata is
defined within objects we generate metadata for. Presently there are
string literals defined and grouped into an unvalidated object returned
from the child providers. This changes makes progress toward having
stronger guarantees that are also type checked.

This PR cleans up the base class and moves toward having these defined
as properties of providers classes, which they are. To keep the PR from
being too large it makes this change to the classname only.

This work precedes initialization internal metadata into a Pydantic
object directly.

src/fmu/dataio/aggregation.py

@@ -243,7 +243,9 @@ def _generate_aggrd_metadata(
             "model": template["fmu"]["model"],
         }
         etemp = dataio.ExportData(config=config, name=self.name)
-        objdata = objectdata_provider_factory(obj=obj, dataio=etemp).get_objectdata()
+
+        objectdata_provider = objectdata_provider_factory(obj=obj, dataio=etemp)
+        objdata = objectdata_provider.get_objectdata()
 
         template["tracklog"] = [generate_meta_tracklog()[0].model_dump(mode="json")]
         template["file"] = {
@@ -260,8 +262,8 @@ def _generate_aggrd_metadata(
             template["data"]["name"] = self.name
         if self.tagname:
             template["data"]["tagname"] = self.tagname
-        if objdata.bbox:
-            template["data"]["bbox"] = objdata.bbox
+        if bbox := objectdata_provider.get_bbox():
+            template["data"]["bbox"] = bbox.model_dump(mode="json", exclude_none=True)
 
         self._metadata = template