refactor(engine): allow scripts to register experiments (#860)

See https://github.com/ooni/probe/issues/2216

internal/registry/httphostheader.go

@@ -0,0 +1,22 @@
+package registry
+
+//
+// Registers the `httphostheader' experiment.
+//
+
+import (
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/httphostheader"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func init() {
+	allexperiments["http_host_header"] = &Factory{
+		build: func(config interface{}) model.ExperimentMeasurer {
+			return httphostheader.NewExperimentMeasurer(
+				*config.(*httphostheader.Config),
+			)
+		},
+		config:      &httphostheader.Config{},
+		inputPolicy: model.InputOrQueryBackend,
+	}
+}