安裝函式庫：
pip install line-bot-sdk
Event:
- MessageEvent 訊息
- TextMessage(text)
- ImageMessage(original_content_url, preview_image_url)
- StickerMessage(package_id, sticker_id)
- FileMessage
- VideoMessage
- FollowEvent
- UnFollowEvent
- JoinEvent
- LeaveEvent
- MemberJoinedEvent
- MemberLeftEvent
linebot會優先把request物件解析，包裝成event, 然後通過通過物件的屬性進行存取