[FEATURE] Filter valid entities when using the `ls` command

[hasan7n]

medperf <entity> ls currently doesn't have an option to only show entities that have is_valid flag as True. This feature was requested from BraTS2023 organizers.

The solution would be to have an option (e.g. medperf <entity> ls --valid) that lists only valid entities.

[aristizabal95]

This filtering is client-side

[VukW]

merge master here and see what's been changed during this half of year