Support for OpenAPI v3

[mnojek]

The APIModel class only supports OpenAPI version 2, while prance that is used for parsing the API is able to analyze also v3. We need to add different methods for parsing swagger v3.

[bhirsz]

The simplest out of the box support for OpenAPI v3 can be done if we update swagger["definitions"] to swagger["components"]["schemas"] in our code. There could be a lot of other stuff missing but it was enough to parse OpenAPi v3 file generated from FastApi example application.

I will update our code