Support Flex Message Update 2

[tkgauri]

News: https://developers.line.biz/en/news/2020/10/08/flex-message-update-2-released/

1. • Changed the maximum number of bubbles that can be included in a carousel
2. • Box component has justifyContent and alignItems property
3. • Use linear gradient as the background image for a box component by setting its background.type property equal to linearGradient.
4. • Allowed an empty array to be specified as the value of a box's contents property
5. • Added support for animated images
6. • Changed how sizes are specified
   1. • Image component
   2. • Icon, text, and span components
7. • Allowed text to automatically shrink to fit a component's width - Text and Button
8. • Spacer has been discontinued