PHPC-73 disabled cloning for all extension classes, but we later added cloning for BSON objects in PHPC-1230. Since ReadConcern, ReadPreference, and WriteConcern are also value objects, we can consider supporting cloning for them as well.

While CursorId, WriteConcernError, and WriteError are also value objects, I'm not considering them here as users are unlikely to ever create those objects directly. It makes more sense to defer their support until the feature is requested (if ever).