Add as_task_clust() converter

[mllg]

We want to avoid new users to instantly be confronted with R6 during task construction. Instead, they can call as_task_clust(<data.frame>).

Also, as_prediction_clust() allows converting back data tables generated by as.data.table.PredictionClust().

[pfistfl]

We could, in this context also try to get mlr3's convert_task working with cluster tasks.
This is currently impossible since convert_task tries to set a target, which is not allowed in for cluster tasks..

[damirpolat]

This looks good, thank you!

[damirpolat]

We could, in this context also try to get mlr3's convert_task working with cluster tasks.
This is currently impossible since convert_task tries to set a target, which is not allowed in for cluster tasks..

Not sure if that would be necessary? Also, that would mean that mlr3 relies on mlr3cluster. Would that be acceptable?