[REQ] Better enum support in rust

Is your feature request related to a problem? Please describe.

Whilst I quite like this project, i found a problem arises with the rust enums. This arises when you have error enums which have no fields, such as in the pet store example:

responses:
  400:
    description: Invalid ID supplied
  404:
    description: Pet not found
  405:
    description: Validation exception

With the error codes 400, 404 and 405 since they have no content they will fail to convert into the enumerated response generated and so instead will have to be accessed via a response error.

Describe the solution you'd like

Rather than using #[serde(untagged)] I think it might be better to work based of status code. It might also be good to implement Display and Error on the error type using the description field.

Describe alternatives you've considered

So far in my project I have just been using the ResponseError sub variant instead, but this is clunky. Another option would be to separate the functionality mentioned above into an additional feature.

In addition a trait could be made such as DesieralizeWithStatusCode to ease the project.

Anyhow, many thanks with this project, it is so much easier than manually creating a reqwest library!

Start with the generated Rust response enums for the pet store example and examine how serde untagged handling treats status-code responses with no content. Compare that behavior with the proposed status-code-based deserialization, and consider the requested Display and Error implementations using the description field. Done means empty error responses deserialize into the generated enum without requiring ResponseError, with behavior covered for the 400, 404, and 405 cases.