Dictionary field

Dictionary (schemaless) field is first most needed field for jsonmodels:
- https://github.com/jazzband/jsonmodels/pull/84
- https://github.com/jazzband/jsonmodels/pull/141

And there are good reasons to having one - for example we could have a list of tags like `{"tag1": "value1", "tag2": 123, ..., "tagn": None}`.

(This is part of bigger problem -> https://github.com/jazzband/jsonmodels/issues/143)

The question is how to implement it - we can choose either of those two pull requests as base, or came up with something different.

Things to consider:
- how deep the constraints should go? We can either declare 'object', or some valid types, or even TypedDict?