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_file.go

@@ -52,6 +52,8 @@ func init() {
 	engine.RegisterResource(httpServerFileKind, func() engine.Res { return &HTTPServerFileRes{} })
 }
 
+var _ HTTPServerGroupableRes = &HTTPServerFileRes{} // compile time check
+
 // HTTPServerFileRes is a file that exists within an http server. The name is
 // used as the public path of the file, unless the filename field is specified,
 // and in that case it is used instead. The way this works is that it autogroups