feat: add function to extract information from a model

[be-marc]

No description provided.

[be-marc]

Closes #1013 and #899

[be-marc]

learner = lrn("classif.rpart")
task = tsk("iris")
resampling = rsmp("cv", folds = 3)

extractor = function(learner) list(selected_features = setdiff(learner$model$frame$var, "<leaf>"))

rr = resample(task, learner, resampling, extractor = extractor)
rr$learners[[1]]$state$selected_features

[berndbischl]

it is not doced where the result of extract is stored

[berndbischl]

also always doc the signature of the function

[berndbischl]

also there should be a use case example and test. this should include a benchmark.

a user needs to somehow practilly gain access to the extracted stuff at the end

[be-marc]

We will implement #1214 instead.