feat(torsf): experiment that bootstraps tor using snowflake (#387)

The current implementation assumes the user has already installed tor
on the current system. If tor is not present, the experiment fails.

This is meant to be the first version of this experiment.

We are going to add more functionality in subsequent revisions of
this experiment, once we've collected more feedback.

Reference issue: https://github.com/ooni/probe/issues/1565.

Here's the spec PR: https://github.com/ooni/spec/pull/218.

Here's the issue tracking future work: https://github.com/ooni/probe/issues/1686

internal/engine/model/experiment.go

@@ -55,7 +55,7 @@ type ExperimentMeasurer interface {
 	// measurement, and experiment calbacks. This method should only
 	// return an error in case the experiment could not run (e.g.,
 	// a required input is missing). Otherwise, the code should just
-	// set the relevant OONI error inside of the measurmeent and
+	// set the relevant OONI error inside of the measurement and
 	// return nil. This is important because the caller may not submit
 	// the measurement if this method returns an error.
 	Run(