Adds new NURBS knot mode NURBS_KNOT_MODE_FREE. Knots are stored in
`CurvesGeometry::custom_knots`. Knot offsets binding them to their
curves are calculated at runtime and held in a cache. This commit adds
custom knots support to OBJ import. It is the only way create such
curves for now. Legacy curve's tools don't support this knot mode,
thus on any modification knots get regenerated and whole shape changes.
If to convert imported legacy curve to new curves, point deletion,
duplicate and extrude preserve custom knots.
Rel #99891
Pull Request: https://projects.blender.org/blender/blender/pulls/130132
8f6af72a3f