Geometry: add .attributes in the Python API for Mesh, Hair and Point Cloud

This puts all generic float/int/vector/color/string geometry attributes in a new
.attributes property. For meshes it provides a more general API for existing
attributes, for point clouds attributes will be used as an essential part of
particle nodes.

This patch was implemented by @lichtwerk, with further changes by me. It's
still a work in progress, but posting here to show what is going on and for
early feedback.

Ref T76659

Differential Revision: https://developer.blender.org/D8200

source/blender/makesrna/intern/rna_pointcloud.c

@@ -156,6 +156,8 @@ static void rna_def_pointcloud(BlenderRNA *brna)
   RNA_def_property_collection_funcs(
       prop, NULL, NULL, NULL, NULL, NULL, NULL, NULL, "rna_IDMaterials_assign_int");
 
+  rna_def_attributes_common(srna);
+
   /* common */
   rna_def_animdata_common(srna);
 }