Adding recursion on child of List Field

[Seraf]

This pull request add recursively the metadata of the child in a list field.

For a serializer like this :

class PowerSerializer(serializers.Serializer):
    groups = serializers.ListField(child=serializers.IntegerField(), default=[], required=False)

You will have this metadata:

"actions": {
    "POST": {
      "groups": {
        "type": "list",
        "required": false,
        "read_only": false,
        "label": "Groups",
        "child": {
          "type": "integer",
          "required": true,
          "read_only": false
        }
      }
    }
  }

I included the test directly in a existing test. Don't know if it was necessary to create a new one.
In the test provided, I use the recursion to be sure it works correctly.