Add OpenAPI support

[erakli]

Hello!

I've recently started to use mountebank in my Python project and found that there are no usable async clients. While it is super easy to write the new one, I've decided to make OpenAPI spec file. By doing this I could easily generate async client with my existing template.

Describing spec file by hand is not a big deal, but I tried to add autogen to mountebank API. Unfortunately, I'm not very convenient with JS and specifically express-js, so I failed with it.

If you add OpenAPI to the project then there would be much more clients in many languages.

I could share my spec file. While it covers API only in some of the parts it would be a great starting point.

[brunns]

Off topic perhaps, but I'd be happy to discuss making https://github.com/brunns/mbtest async friendly if that's feasible and useful.

[erakli]

@brunns I'm using your lib in my project with async code also. In the project I've added async realization of main classes. While they could be incorporated in mbtest I'm sure that most of hardlifting could be done with auto-gen client. So in first place I suppose that there need to be OpenAPI support from montebank :)