Consider implement jwt.Cliams for the structure in path appstore

[leeshun]

I see the struct in the path appstore/api have implement jwt.Claims, and these struct can just call jwt.ParseWithClaims. But the struct in the path appstore can only use jwt.Parse and we should write so many convert code to get data, such as

        token := jwt.Token{}
	if err := client.ParseNotificationV2(tokenStr, &token); err != nil {
		t.Errorf("expected an error to be nil since the tokenStr is legal")
	}
	claims, ok := token.Claims.(jwt.MapClaims)
	if !ok {
		t.Errorf("claim should be jwt.MapClaims")
	}

[richzw]

@leeshun , the ParseNotificationV2WithClaim had been added. Please let me know if there is anything more.