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Geometry Nodes: Add Attribute interpolation for polygon domains

This commit adds interpolation to and from attribute on the polygon
domain. Interpolation is done automatically when a node uses attributes
on two different domains. The following are the new interpolations and
corresponding simple test cases:
- **Point to Polygon**: Painting the shade smooth attribute in weight
paint mode
- **Polygon to Point**: Moving points along a normal based on the
material index
- **Polygon to Corner**: Scaling a UV map with the material index
before sampling a texture

{F9881516}

This is also necessary for an improved implementation of the `normal`
attribute.

Differential Revision: https://developer.blender.org/D10393