run_paramtest should be able to generate parameters depending on the tasks

[sebffischer]

Some learners require (such as priority lass from mlr3extralearners) require to set a parameter to a specific value, that depends on properties of the tasks (in this case, the number of features).

Currently the run_autotest function does not really support this.

Suggestion: add a parfn argument that takes in arguments task, learner and outputs a list with the new parameters.