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Geometry Nodes Curves: Expose first builtin point attributes

This commit exposes the first spline control point attributes. The
implementation incorporates the attributes into the virtual array
system, providing efficient methods to flatten the data into a
contiguous array and to apply changes from a flattened array. This
is only part of the eventual goal, which includes changes to run
attribute nodes separately for each spline to completely avoid copying.

So far `tilt` and `radius`, the two generic attributes common to
all spline types, are implemented. The more complex `position`
attribute is also added. It requires some special handling for Bezier
splines, where the control point handles need to be moved along with
the control points. To make that work I also added automatic handle
recalculation to the Bezier spline.

Differential Revision: https://developer.blender.org/D11187