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Geometry Nodes: Expose vertex normals as an attribute

This attribute exposes mesh vertex normals as a `vertex_normal`
attribute for use with nodes. Since the normal vector stored in
vertices is only a cache of data computable from the surrounding faces,
the attribute is read-only. A proper error message for attempting to
write this attribute is part of T85749. A write-only normal attribute
will likely come later, most likely called `corner_normal`.

The normals are recomputed before reading if they are marked dirty.
This involves const write-access to the mesh, protected by the mutex
stored in `Mesh_Runtime`. This is essential for correct behavior after
nodes like "Edge Split" or nodes that adjust the position attribute.

Ref T84297, T85880, T86206

Differential Revision: https://developer.blender.org/D10541