Create function `as_point()` that converts sample- or quantile-based forecasts into point forecasts

[nikosbosse]

Meta-issue: #433. Next issue: #434

We would like to be able to compute point forecasts for quantile- and sample-based forecasts. The cleanest way to do this would be to have a function that converts the forecast into a point forecasts and then you can score it again. Something like:

example_continuous |>
  as_point(statistic = "median") |>
  score()

Additional considerations: For sample-based forecasts that would be quite similar to summarise_scores(data, fun = median). For quantile scores passing in your own function maybe makes less sense as there is not much you can do except the median. (see discussion in #340.

[seabbs]

As suggested I think this is good to push into nice to have vs do now.