engine: resources: Add some compile time checks for groupers

These can "break" silently and not autogroup if we change the resource
and it no longer fulfills the interface. Add this compile time check to
prevent that.

engine/resources/http_server_ui.go

@@ -70,6 +70,8 @@ func init() {
 	engine.RegisterResource(httpServerUIKind, func() engine.Res { return &HTTPServerUIRes{} })
 }
 
+var _ HTTPServerGroupableRes = &HTTPServerUIRes{} // compile time check
+
 // HTTPServerUIGroupableRes is the interface that you must implement if you want
 // to allow a resource the ability to be grouped into the http server ui
 // resource. As an added safety, the Kind must also begin with