[VisibilityController] Stackable ScanLabelGenerators

The ScanLabelGenerator is used by the VisibilityController to assemble the effective label set for a user in the RPC context before processing any request. Currently only one implementation of this interface can be installed, although which implementation to use can be specified in the site file. Instead it should be possible to stack multiple implementations of this component the same way we do coprocessors, installed with explicit priority with ties broken by a counter, where those implementations installed later in the chain have an opportunity to modify the pending effective label set.