Add ROC metric to AnomalyModule

[dreaquil]

Is your feature request related to a problem? Please describe.
Currently only AUROC and F1 for image and pixel are measured. I think it would be useful to (optionally) include the full ROC.

Describe the solution you'd like
It would be useful to have the ROC metric included as standard or optional.

Note
ROC is a bit problematic as a metric because it is currently not possible to simply LightningModule.log or log_dict it.

[ashwinvaidya17]

This PR (#429) adds plots for ROC curves. It should address your need