add plugin http handler (#7289)

plugin/plugintest/hooks.go

@@ -1,6 +1,8 @@
 package plugintest
 
 import (
+	"net/http"
+
 	"github.com/stretchr/testify/mock"
 
 	"github.com/mattermost/platform/plugin"
@@ -19,3 +21,7 @@ func (m *Hooks) OnActivate(api plugin.API) error {
 func (m *Hooks) OnDeactivate() error {
 	return m.Called().Error(0)
 }
+
+func (m *Hooks) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	m.Called(w, r)
+}