[ENH] forecasting benchmarking task experiment

[fkiraly]

This PR adds a SktimeForecastingTask, which defines a full benchmarking run for a forecaster that is passed later in _evaluate.

This object could be used as a "task" in the sktime ForecastingBenchmark.

Draft for discussion and reviewing the design:

• it is quite similar to and partially duplicative with SktimeForecastingExperiment which is used in tuning. How should we deal with the similarity and intersection?
  • we could merge into a single class, depending on whether forecaster gets passed or not. Not sure where that leads though
• is this a possible 1:1 dropin (or almost) for the task object in sktime?