PLT-7407: Back-end plugins (#7409)

* tie back-end plugins together

* fix comment typo

* add tests and a bit of polish

* tests and polish

* add test, don't let backend executable paths escape the plugin directory

plugin/plugintest/hooks.go

@@ -22,6 +22,10 @@ func (m *Hooks) OnDeactivate() error {
 	return m.Called().Error(0)
 }
 
+func (m *Hooks) OnConfigurationChange() error {
+	return m.Called().Error(0)
+}
+
 func (m *Hooks) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	m.Called(w, r)
 }