Mesh: Move sharp edge flag to generic attribute

Move the `ME_SHARP` flag for mesh edges to a generic boolean
attribute. This will help allow changing mesh edges to just a pair
of integers, giving performance improvements. In the future it could
also give benefits for normal calculation, which could more easily
check if all or no edges are marked sharp, which is helpful considering
the plans in T93551.

The attribute is generally only allocated when it's necessary. When
leaving edit mode, it will only be created if an edge is marked sharp.
The data can be edited with geometry nodes just like a regular edge
domain boolean attribute.

The attribute is named `sharp_edge`, aiming to reflect the similar
`select_edge` naming and to allow a future `sharp_face` name in
a separate commit.

Ref T95966

Differential Revision: https://developer.blender.org/D16921

source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc

@@ -192,12 +192,15 @@ void OBJMesh::ensure_mesh_normals() const
 
 void OBJMesh::calc_smooth_groups(const bool use_bitflags)
 {
+  const bool *sharp_edges = static_cast<const bool *>(
+      CustomData_get_layer_named(&export_mesh_->edata, CD_PROP_BOOL, "sharp_edge"));
   poly_smooth_groups_ = BKE_mesh_calc_smoothgroups(mesh_edges_.data(),
                                                    mesh_edges_.size(),
                                                    mesh_polys_.data(),
                                                    mesh_polys_.size(),
                                                    mesh_loops_.data(),
                                                    mesh_loops_.size(),
+                                                   sharp_edges,
                                                    &tot_smooth_groups_,
                                                    use_bitflags);
 }