KHR_animation_pointer support for glTF import and export

[emperorofmars]

The animation system in glTF is very limited.
The KHR_animation_pointer extension proposal makes pretty much any glTF property, including those in extensions, animatable.
Supporting this extension for both import and export would allow for Godot to become a powerful universal tool for animation in videogames.

As not everything works the same between different engines and tools, a way to remap/retarget animations on import and export would be very powerful.
For example, a file exported from Unity with animations targeting a ParentConstraint's values would not quite work in Godot.
To represent a Unity style ParentConstraint would require a custom node and therefore a different animation path.

A import toggle wether animation paths that cannot be 'retargeted' should be thrown away, or kept would also be nice.
If someone is creating an application purely in Godot, those would just be a waste of space, but if the glTF file is intended to be edited in Godot and exported again, those should be kept.