reading file and putting json file in Nested structure in Go lang with appending extra data

I have nested structure one structure coming from reading file and second I'm providing my self and I have to stdout the file. I got the code but I'm having hard time with syntax and logic I'm new to go lang.(the data structure supposed to come from the file)

The program reads the file, but then substitutes something else in for data. Just put the data in the value marshaled to the output.

type Event struct {
	Specversion string    `json:"specversion"`
	ID          uuid.UUID `json:"id"` /// uuid
	Source      string    `json:"source"`
	Type        string    `json:"type"`
	Time        time.Time `json:"time"`
	Data        string    `json:"data"` /* <= change to string */
}

func main() {

	if len(os.Args) < 1 { //checking for provided argument if the first arg satisfy then use read file
		fmt.Println("Usage : " + os.Args[0] + " file name")
		help()
		os.Exit(1)
	}

	Data, err := ioutil.ReadFile(os.Args[1])
	if err != nil {
		fmt.Println("Cannot read the file or file does not exists")
		os.Exit(1)
	}

	e := Event{
		Specversion: "1.0",
		ID:          uuid.New(),
		Source:      "CSS",
		Type:        "", //  user input -p or -u,
		Time:        time.Now(),
		Data:        string(Data), /* <= stuff data into output value */
	}

	out, _ := json.MarshalIndent(e, "", "  ")
	fmt.Println(string(out))
}

Run it on the playground to get results as requested in the question.

Perhaps the \r\n in the output are the problem. Remove carriage return and line feed from the JSON before encoding the result:

replacer := strings.NewReplacer("\r", "", "\n", "")
Data = []byte(replacer.Replace(string(Data)))

Run it on the playground.

If you intent is to include the JSON as JSON instead of quoting the JSON as string, then use json.RawMessage:

type Event struct {
	Specversion string    `json:"specversion"`
	ID          uuid.UUID `json:"id"` /// uuid
	Source      string    `json:"source"`
	Type        string    `json:"type"`
	Time        time.Time `json:"time"`
	Data        json.RawMessage    `json:"data"` /* <= change to json.RawMessage */
}

func main() {

	if len(os.Args) < 1 { //checking for provided argument if the first arg satisfy then use read file
		fmt.Println("Usage : " + os.Args[0] + " file name")
		help()
		os.Exit(1)
	}

	Data, err := ioutil.ReadFile(os.Args[1])
	if err != nil {
		fmt.Println("Cannot read the file or file does not exists")
		os.Exit(1)
	}

	e := Event{
		Specversion: "1.0",
		ID:          uuid.New(),
		Source:      "CSS",
		Type:        "", //  user input -p or -u,
		Time:        time.Now(),
		Data:        Data, /* <== use data as is */
	}

	out, _ := json.MarshalIndent(e, "", "  ")
	fmt.Println(string(out))
}

Run it on the playground to view nested json. You show a quoted string in the question, but perhaps this output is what you really want.