Add Integration tests for responses

[cortinico]

This commit introduces multiple JUnit tests for sending and receiving
responses using the Swagger Codegen generated models.

Tests are running using the OkHttp MockWebServer and mock responses
have been inlined in the body of the tests.

[macisamuele]

Overall the changes looks good.
I would like to ask, if do-able, to have assertions on how the mock server is actually called.
The tests do check that the decoding of the response is working as expected but we're not doing much with respect to encoding of the request. Eventually doing it in a follow-up would be great considering the extent of the current PR.

[cortinico]

The tests do check that the decoding of the response is working as expected but we're not doing much with respect to encoding of the request. Eventually doing it in a follow-up would be great considering the extent of the current PR.

Agree, I'd rather keep it outside of this PR anyway as it's already pretty big.
I can extend every test to make sure that the requests received by the MockWebServer are properly encoded.

[macisamuele]

Feel free to merge as is and eventually follow up with requests received by the MockWebServer are properly encoded