Allow multiple input/output types in `as.data.table.DictionaryPipeOp`

[advieser]

It would be good if it were possible to specify multiple different allowed task_types to be displayed in input.type.train, input.type.predict, output.type.train, and output.type.predict, when calling the S3 method as.data.table.DictionaryPipeOp for mlr_pipeops.

As far as I can tell, right now this would only be necessary for the upcoming PipeOpEncodePLTree in #861.