Add ContainerField

[xirdneh]

A field that represents a container. The values in the container should be represented by one or more Field instances.

• Accept an items_fields parameter to represent each item. This could be a single Field instance or a tuple. Tuples would be used in fields like mappings to represent key/value pairs. e.g. Dict[str, int] or to represent different possible values, e.g. List[Union[str, int]]