Add support for importing from JSON data files

[rvilar]

We want to add support for importing content via JSON data files and try to take advantage of REST WS.

[rvilar]

Now that we will have CSV, JSON and YAML data providers we have to think in an architecture of submodules.

Wich could be a good structure for that? Plugins maybe?

[rodrigoaguilera]

What is the use case?
So a third party can say "hey, I want to write my default content in xml,xls or whatever".

I feel once we have JSON is not worth the effort to bake a plugin manager but if someone gives a reason for using something different than CSV, YAML or JSON we can look into it.

I think we can even go as far as to deprecate the use of csv because of its plain structure nature.

So to keep this in scope I think we should implement JSON support similarly as YML is being implemented now without extensibility for new formats.

[rodrigoaguilera]

I think this requires more effort than initially was planned. Feel free to reopen on https://www.drupal.org/project/migrate_default_content/issues
if it is still in the roadmap