Geometry Nodes: new Interpolate Curves node

This adds a new `Interpolate Curves` node. It allows generating new curves
between a set of existing guide curves. This is essential for procedural hair.

Usage:
- One has to provide a set of guide curves and a set of root positions for
  the generated curves. New curves are created starting from these root
  positions. The N closest guide curves are used for the interpolation.
- An additional up vector can be provided for every guide curve and
  root position. This is typically a surface normal or nothing. This allows
  generating child curves that are properly oriented based on the
  surface orientation.
- Sometimes a point should only be interpolated using a subset of the
  guides. This can be achieved using the `Guide Group ID` and
  `Point Group ID` inputs. The curve generated at a specific point will
  only take the guides with the same id into account. This allows e.g.
  for hair parting.
- The `Max Neighbors` input limits how many guide curves are taken
  into account for every interpolated curve.

Differential Revision: https://developer.blender.org/D16642

tests/python/CMakeLists.txt

@@ -815,6 +815,7 @@ set(geo_node_tests
   attributes
   curve_primitives
   curves
+  curves/interpolate_curves
   geometry
   instance
   mesh_primitives