Fix ListOrItemField serializer validation

This is a proposed fix for the issue reported in #15.

In ListOrItemField, implement initialize and field_from_native to ensure embedded serializers are validated
In the [current implementation](https://github.com/estebistec/drf-compound-fields/blob/d6a143bbb0aff45a6720538e17644c3e491372d2/drf_compound_fields/fields.py#L100) of `from_native`, we return the passed in data in this case.