Expose the new gizmo plugin system to scripting

[JFonS]

I will write some documentation for this once it's merged and I have some free time :)

Closes #23271

[akien-mga]

Note: labelled "bug" as not having this feature was a regression compared to 3.0.

[Zylann]

Would this help to fix #17499?

[akien-mga]

X11 and server tools=yes fail building:

scene/3d/spatial.cpp: In member function 'void Spatial::update_gizmo()':
scene/3d/spatial.cpp:404:43: error: 'SceneTree::GroupCallFlags' is not a class or namespace
   get_tree()->call_group_flags(SceneTree::GroupCallFlags::GROUP_CALL_REALTIME, SceneStringNames::get_singleton()->_spatial_editor_group, SceneStringNames::get_singleton()->_request_gizmo, this);
                                           ^

[akien-mga]

Thanks!