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_flag.go

@@ -48,6 +48,8 @@ func init() {
 	engine.RegisterResource(httpServerFlagKind, func() engine.Res { return &HTTPServerFlagRes{} })
 }
 
+var _ HTTPServerGroupableRes = &HTTPServerFlagRes{} // compile time check
+
 // HTTPServerFlagRes is a special path that exists within an http server. The
 // name is used as the public path of the flag, unless the path field is
 // specified, and in that case it is used instead. The way this works is that it