No support for nested fields in Schema specification

[Oracking]

Currently, there is no support for nested fields when specifying the request/response schema.

E.g. If I specify a schema like: {"name": "string", "countries": ["string"]}, it causes an invalid swagger to get generated

An invalid swagger is generated for both cases: supplying the schema as a dict or as a Marshmallow schema.

[MDUYN]

I will start a feature branch for this

[noorkham]

what about this ?