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Geometry Nodes: Join dynamic curve attributes in the join geometry node

This commit lets the join geometry node transfer dynamic attributes
to the result, the same way that point cloud and mesh attributes are
joined. The implementation is different though, because of an
optimization implemented for curves to avoid copying splines.

The result attribute is added with the highest priority domain (points
over splines), and the highest complexity data type. If one curve had
the attribute on the spline domain but not others, the point domain
values will be used.

Generally this is a bit lower level than I would have liked this code
to be, but should be efficient, and it's really not too complicated.

Differential Revision: https://developer.blender.org/D11491