engine: pass model.ExperimentArgs to Experiment.Run

[bassosimone]

This issue is a child of ooni/ooni.org#1292. The observation here is that we are passing to the Experiment's Run method individual arguments. This design choice makes it difficult to apply changes such as adding new, possibly-optional arguments to experiments, because we need to refactor each experiment. On the contrary, if we replace all the arguments with a structure containing the arguments, there's less friction and we can more easily apply changes. Once this change has been implemented, we'll be in a better position to pass richer input to experiments.