Internal. If the user authors refinementEnableOverride, drop a hint in customLayerData that we can use to enable/disable the checking for override attributes in the scene delegate (i.e., assuaging Pixar’s concern that even assets that do not opt into per-mesh refinement must check for their presence at load time).

The hint also gives us a numerical value that we can use to maintain backwards compatibility, as it is almost certain that the attributes and the value resolution logic governing this override behavior will be revisted, i.e., also encodable in a more high-level enum like Complexity setting in usdview, which could also be an attribute than lives on any prim, not just meshes, and whose behavior would inherit down namespace.