rdt: config flag for a class to reqeust exclusive cache allocation

Nice to have would be a flag in the config to request/guarantee that a class has exclusive cache allocation.

The behavior would just be a bit blunt (I think): configuration will fail if the class cannot be made exclusive. Reasons might be RDT misconfiguration (in goresctrl rdt config) or that there are other existing classes overlapping (that are not managed by goresctrl)