Add --go-module-name flag to support generating Go module compatible code (#7651)

* Add --go-module-name flag to support generating code for go modules

* Rename echo example folder

* Grammar

* Update readme for go-echo example

* Update readme for go-echo example

* Re-enable go modules after test is done

examples/go-echo/net.fbs

@@ -0,0 +1,11 @@
+include "hero.fbs";
+
+namespace net;
+
+table Request {
+	player: hero.Warrior;
+}
+
+table Response  {
+	player: hero.Warrior;
+}