Import existing OpenApi integration

[maurei]

In a private repository we have experimented with integrating OpenAPI with JsonApiDotNetCore by:

• adding the Swashbuckle SwaggerGen to a JADNC application.
• generating a client using NSwag

We have decided to extract this code into this repository and further generalize the integration from there.

Along with the migration we will introduce a set of test models that reflect the models that were used in the private repository, and with it a test in which we compare the generated OAS with an hardcoded expected value. This will be useful to maintain as long as we haven't introduced dedicated test suites for every OpenApi feature.