[REQ] Derrive Default for generated Rust structs

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

It would be useful to add Default to the list of derived traits here. This would make it simpler to create defaults for types. This would allow a consumer to create defaults of structs like

let default_pet = Pet::default();

This is really helpful when mocking things out, writing tests, etc, where you just need a valid instance to fulfill some list of arguments.

Describe the solution you'd like

Add Default to the list of derived attributes on generated struct models. I am assuming this is possible because all structs are serializable to and from JSON so they must eventually resolve to the set of primitive types that have defaults defined in the Rust standard library.

Describe alternatives you've considered

The alternative is to implement the Default trait per type which adds a lot of boiler plate. For example,

impl Default for Pet {
    fn default() -> Self { 
        Pet {
            // ...
        }
    }
}

Start with modules/openapi-generator/src/main/resources/rust/model.mustache, especially the derived-trait list around line 47. Check how generated Rust struct models handle their fields, then verify that generated examples can use Pet::default() and that the relevant Rust generation tests or compilation checks pass.