Implement MarshalJSON for Flex Components and Containers

[acoshift]

Thank you for the hard work for Line Bot SDK for Go 👍

This PR changes

• Implement MarshalJSON for Flex Components, and Containers
• Breaking Change Remove field Type from Flex Components, and Containers

I think it very confusing to put the same Type to the named struct.

[sugyan]

Thanks for your contribution.
But... we don't assume much the use case of writing each struct of containers and components. It is convenient that use UnmarshalJSON method from JSON data which is generated by the simulator.
https://developers.line.biz/console/fx/
So, we think that it is not needed to implement MashalJSON.

Or how about adding only the implementation of MarshalJSON so as not to make a destructive change?
For example, without deleting the definition of the Type element, when the correct value is not set, MarshalJSON automatically complements it inside it.

[acoshift]

Hi,
I change the PR to implement only MarshalJSON without breaking change as your suggest.

My use-case is to generate flex message dynamically from backend.
I think it's harder to unmarshal from json and replace data/elements.

[k2wanko]

LGTM