[4.x] Expose NavigationServer3D to GDExtension

[DarkKilauea]

Fixes: #36091

Supersedes: #65562

Adds the ability to register multiple implementations of NavigationServer3D and allow the user to select the one they want to use in their project.

Also exposes NavigationServer3D to GDExtension to make it possible for plugins to replace the default implementation.

NavigationServer2D is left alone, as it is a wrapper around NavigationServer3D and separating it will be a lot of work. Might be something worth considering for the future if there is demand.

Things still WIP:

• _query_path_extension has a const pointer parameter whose type is not being picked up correctly.
• Document NavigationServer3DExtension.
• Test compiling out the Godot Navigation module.

[DarkKilauea]

Fixed the issue with the missing type for the params parameter of _query_path_extension. Also added a dummy implementation of NavigationServer3D to stop Godot from crashing outright if the navigation module is disabled during compilation. I can now confirm that Godot builds and launches without the built-in navigation module.

Currently working on fixing an issue with the generated C++ header for NavigationServer3DExtensionPathQueryResult, then we should be pretty close to being ready for final reviews and a merge.

[DarkKilauea]

Marking this as ready for review. I still need to add some documentation, but the code should be ready to go now.

[DarkKilauea]

All documentation should be up to date now.

[DarkKilauea]

Rebased to account for changes from #67111

[DarkKilauea]

Rebased to account for changes to copyright headers and ProjectSettings::set_custom_property_info