Geometry Nodes: add Attribute Mix node

This node can be used to mix two attributes in various ways.
The blend modes are the same as in the MixRGB shader node.

Differential Revision: https://developer.blender.org/D9737

Ref T82374.

release/scripts/startup/nodeitems_builtins.py

@@ -484,6 +484,7 @@ geometry_node_categories = [
         NodeItem("GeometryNodeRandomAttribute"),
         NodeItem("GeometryNodeAttributeMath"),
         NodeItem("GeometryNodeAttributeFill"),
+        NodeItem("GeometryNodeAttributeMix"),
     ]),
     GeometryNodeCategory("GEO_COLOR", "Color", items=[
         NodeItem("ShaderNodeValToRGB"),