GizmoPrimitiveNd for external primitives

## What problem does this solve or what need does it fill?

The current definition for `GizmoPrimitive2d` and `GizmoPrimitive3d` do not allow implementing gizmos for primitives outside of `bevy_gizmos` since both `GizmoPrimitiveNd` and `Gizmos` are defined in `bevy_gizmos` and implementing `GizmoPrimitiveNd` for `Gizmos` outside `bevy_gizmos` would violate coherency.

This is annoying when trying to implement gizmos for custom primitives in games since you basically have to replicate the `GizmoPrimitiveNd` trait in your own project.