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/hooks.go

@@ -12,6 +12,9 @@ type Hooks interface {
 	// use the API, and the plugin will be terminated shortly after this invocation.
 	OnDeactivate() error
 
+	// OnConfigurationChange is invoked when configuration changes may have been made.
+	OnConfigurationChange() error
+
 	// ServeHTTP allows the plugin to implement the http.Handler interface. Requests destined for
 	// the /plugins/{id} path will be routed to the plugin.
 	//