Geometry Nodes: Add Attribute Convert node

The Attribute Convert node provides functionality to change attributes
between different domains and data types. Before it was impossible to
write to a UV Map attribute with the attribute math nodes since they
did not output a 2D vector type. This makes it possible to
"convert into" a UV map attribute.

The data type conversion uses the implicit conversions provided by
`\nodes\intern\node_tree_multi_function.cc`.

The `Auto` domain mode chooses the domain based on the following rules:
1. If the result attribute already exists, use that domain.
2. If the result attribute doesn't exist, use the source attribute domain.
3. Otherwise use the default domain (points).

See {T85700}

Differential Revision: https://developer.blender.org/D10624

release/scripts/startup/nodeitems_builtins.py

@@ -485,6 +485,7 @@ geometry_node_categories = [
         NodeItem("GeometryNodeAttributeRandomize"),
         NodeItem("GeometryNodeAttributeMath"),
         NodeItem("GeometryNodeAttributeCompare"),
+        NodeItem("GeometryNodeAttributeConvert"),
         NodeItem("GeometryNodeAttributeFill"),
         NodeItem("GeometryNodeAttributeMix"),
         NodeItem("GeometryNodeAttributeProximity"),