Allow rendering entities providing a view mode.

The idea is to leverage Drupal's configuration capabilities to specify which fields to render and how to render them using the field formatters.

I think it's *great* to have the fine grain control that we have now on how to build the output, but we're core missing a lot from Drupal core by not using the entity view modes.

## Goals:
  * Minimal configuration. Probably without even needing a custom class that extends from `RestfulNodeBase`, `RestfulEntityBase`, …
  * Generating the **exact** desired output **without code**.
  * The only rendering limitations should be those inherited from rendering a view mode.

## Suggested approach:
  * Provide a `'view_mode'` configuration key in the plugin definition.
  * Provide a mapping config array between the keys in the rendered view mode and the desired output.
    - This mapping would be a replace for the default field info method, from there on everything else should be **the same** from the implementation standpoint.
    - This mapping should be seen as plugin configuration, not as code.
    - This mapping should allow path nesting. This of a view mode that has an ER field and renders the referenced entity with only its body. We would do `'field_referenced_node' => 'referenced::body::value'`. That could yield:
```javascript
{
  "referenced": {
    "body": {
      "value": "Lorem ipsum dolor…"
    }
  }
}
```
> Anyway, this could be optional. If a field that exists in the display isn't present here in the array we can assume the public field name to be str_replace('field_', '', $field_name)

I was debating about this as well. I don't like the idea of doing these automated things, I like enforcing being explicit with what you expose. That being said, I think that it would be pretty cool to have a zero-conf way to expose your resources with RESTful.

My take, if we want to be explicit about it leave it in here. If we want to break the pattern and go for the zero-conf approach, then make it contrib.