Create E2E test framework

[bhirsz]

Closes #2

Now it's possible to:

• define the API using fast API
• generate spec for API
• use roboswag generate to generate test libraries
• start API server
• run robot tests using generated test libraries

tests/e2e/simple_v3/test_api.py file shows short example of the defined API and tests for it - everything else is done automatically for us.

We're missing documentation for testing but I will add it in the (close) future - there might be additonal changes / improvements that would require rewritting parts of the docs.

Also fixed:

• description field is not used anymore for validation (Closes #56 ). Instead, if the response does not have schema we are adding proposed validation in comment with TODO marker
• retrieve response schema for openAPI v3