Add constrained_cross_entropy metric

[GoncaloArs]

When using constrained_cross_entropy as an objective, FairGBM constrained_cross_entropy tries to use it as a metric (to report training progress). The issue here is that, currently, no such metric exists.

Everything works fine since a metric is not necessary for instantiating or training a FairGBM object, but it would be nice to have a proper metric.