refactor: pass experiment arguments using a struct (#983)

Closes https://github.com/ooni/probe/issues/2358.

internal/tutorial/experiment/torsf/chapter02/README.md

@@ -117,10 +117,10 @@ chapters, finally, we will modify this function until it is a
 minimal implementation of the `torsf` experiment.
 
 ```Go
-func (m *Measurer) Run(
-	ctx context.Context, sess model.ExperimentSession,
-	measurement *model.Measurement, callbacks model.ExperimentCallbacks,
-) error {
+func (m *Measurer) Run(ctx context.Context, args *model.ExperimentArgs) error {
+	_ = args.Callbacks
+	_ = args.Measurement
+	sess := args.Session
 ```
 As you can see, this is just a stub implementation that sleeps
 for one second and prints a logging message.

internal/tutorial/experiment/torsf/chapter02/main.go

@@ -54,7 +54,12 @@ func main() {
 		MockableLogger:  log.Log,
 		MockableTempDir: tempdir,
 	}
-	if err = m.Run(ctx, sess, measurement, callbacks); err != nil {
+	args := &model.ExperimentArgs{
+		Callbacks:   callbacks,
+		Measurement: measurement,
+		Session:     sess,
+	}
+	if err = m.Run(ctx, args); err != nil {
 		log.WithError(err).Fatal("torsf experiment failed")
 	}
 	data, err := json.Marshal(measurement)

internal/tutorial/experiment/torsf/chapter02/torsf.go

@@ -93,10 +93,10 @@ func (m *Measurer) ExperimentVersion() string {
 // minimal implementation of the `torsf` experiment.
 //
 // ```Go
-func (m *Measurer) Run(
-	ctx context.Context, sess model.ExperimentSession,
-	measurement *model.Measurement, callbacks model.ExperimentCallbacks,
-) error {
+func (m *Measurer) Run(ctx context.Context, args *model.ExperimentArgs) error {
+	_ = args.Callbacks
+	_ = args.Measurement
+	sess := args.Session
 	// ```
 	// As you can see, this is just a stub implementation that sleeps
 	// for one second and prints a logging message.