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Geometry Nodes: Add "normal" attribute for face normals

This commit adds a `normal` attribute on the polygon domain. Since
normal data is derived data purely based off of the location of each
face's vertices, it is exposed as a read-only attribute. After
rB80f7f1070f17, this attribute can be interpolated to the other domains.

Since this attribute is a special case compared to the others, the
implementation subclasses `BuiltinAttributeProvider`. It's possible
there is a better way to abstract this. Something else might also
become apparent if we add similar read-only attributes.

See rB2966871a7a891bf36 for why this is preferred over the previous
implementation.

Differential Revision: https://developer.blender.org/D10677