refactor: refactoring of code

Add graceful shutdown server

main.go

@@ -15,7 +15,9 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 */
-import "github.com/jkaninda/goma-gateway/cmd"
+import (
+	"github.com/jkaninda/goma-gateway/cmd"
+)
 
 func main() {