[Improvement] Separate spawner and object type concepts.

[alexmillane]

Issue Summary

Suggestion to move SPAWNER out to ObjectType.

Issue Details

• We have mixing of concerns in ObjectType (in file object_base.py) between BASE/RIGID/ARTICULATION, that denote the type of object, and SPAWNER which denotes that this object, which can be any of the types above, should use a user-specified spawn function.
• These two concepts should be independent. The type of the object, and the optional presence of a user-specified spawner.
• Right now, all SPAWNER type objects, i.e. objects with custom spawn functions, have to generate a object config of type a AssetBaseCfg.
• This is starting to cause issues where we have objects that require custom spawn functions, but want to be rigid objects, for example. See the object Sphere, for example, in the object_library.py, which is forced to hack around this by overriding _generate_spawner_cfg.

Fix suggestion.

• Suggested to fix by removing SPAWNER from the ObjectType, and treating the concept of having a custom spawn function separately to the issue of what type the object is.