Obtaining structs and binding structs to and from API?

The godot API supports structs.
A prime example is PhysicsServer3DExtension — Godot Engine (stable) documentation in English

How can I bind a struct to the api with GDExtension? Is that possible?
Likewise, how can I make my game use the extension physics server over the default so I can use the struct outputs instead of dictionaries in c++?