Add initial version of schema endpoint generation

[pquentin]

I'm using a simplified version of the Kibana OpenAPI for one endpoint. Simplifications include:

• I replaced additionalProperties: {} with additonalProperties: true manually. (I initially thought it was a Kibana bug, but the two are in fact equivalent: https://swagger.io/docs/specification/v3_0/data-models/dictionaries/#free-form-objects. I intend to fix this later)
• I only handle the 200 error response, and can't handle AnyOf where multiple response are possible.

That said, I'm able to generate a schema.json file (that can be used to generate client) from this simplified OpenAPI representation. This is tested using https://insta.rs/.

[Anaethelion]

Overall looks good, could you add some comments on the endpoints.rs file? There's a lot of pattern matching and its not always obvious what each does.