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

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

internal/engine/experiment/stunreachability/stunreachability.go

@@ -73,10 +73,10 @@ var errStunMissingPortInURL = errors.New("stun: missing port in URL")
 var errUnsupportedURLScheme = errors.New("stun: unsupported URL scheme")
 
 // Run implements ExperimentMeasurer.Run.
-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 {
+	callbacks := args.Callbacks
+	measurement := args.Measurement
+	sess := args.Session
 	tk := new(TestKeys)
 	measurement.TestKeys = tk
 	registerExtensions(measurement)