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…"
    }
  }
}
```
Same as in https://github.com/Gizra/restful/pull/302#discussion_r21586467