This patch exposes the "Shade Smooth" value as an attribute. This
setting is exposed as a check-box in the mesh data properties, but
the value is actually stored for every face, allowing some faces
to be shaded smooth with a simple per-face control.

One bonus, this allows at least a workaround to the lack of control
of whether meshes created by nodes are shaded smooth or not: just use
an attribute fill node.