Add a new normal mode called "Custom" which directly interpolates
a "custom_normal" attribute to the evaluated points for the final
normal. Extend the "Set Curve Normal" node with this mode and
give it the ability to set the custom normal value.
This is intentionally a very basic implementation of custom normals.
In particular, the storage is not rotation invariant. So the normals
are expected to be set procedurally at the end of the modifier stack.
On the other hand, it is very easy to understand and explain.
Pull Request: https://projects.blender.org/blender/blender/pulls/116066
f63a7c1ee9