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_proxy.go

@@ -65,6 +65,8 @@ func init() {
 	engine.RegisterResource(httpServerProxyKind, func() engine.Res { return &HTTPServerProxyRes{} })
 }
 
+var _ HTTPServerGroupableRes = &HTTPServerProxyRes{} // compile time check
+
 // HTTPServerProxyRes is a resource representing a special path that exists
 // within an http server. The name is used as the public path of the endpoint,
 // unless the path field is specified, and in that case it is used instead. The