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/client/client.go

@@ -0,0 +1,62 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+
+	"echo/hero"
+	"echo/net"
+
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+func RequestBody() *bytes.Reader {
+	b := flatbuffers.NewBuilder(0)
+	r := net.RequestT{Player: &hero.WarriorT{Name: "Krull", Hp: 100}}
+	b.Finish(r.Pack(b))
+
+	// Encode builder head in last 4 bytes of request body
+	buf := make([]byte, 4)
+	flatbuffers.WriteUOffsetT(buf, b.Head())
+	buf = append(b.Bytes, buf...)
+
+	return bytes.NewReader(buf)
+}
+
+func ReadResponse(r *http.Response) {
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		fmt.Printf("Unable to read request body: %v\n", err)
+		return
+	}
+
+	// Last 4 bytes is offset.
+	off := flatbuffers.GetUOffsetT(body[len(body)-4:])
+	buf := body[:len(body) - 4] 
+
+	res := net.GetRootAsResponse(buf, off)
+	player := res.Player(nil)
+	
+	fmt.Printf("Got response (name: %v, hp: %v)\n", string(player.Name()), player.Hp())
+}
+
+func main() {
+
+	body := RequestBody()	
+	req, err := http.NewRequest("POST", "http://localhost:8080/echo", body)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	client := http.DefaultClient
+	resp, err := client.Do(req)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	ReadResponse(resp)
+}