[rust-server] Add mechanism to be able to provide additional derives

Description

This is a vague feature idea for rust-server. I may have a look at implementing some day.

Currently, rust-server derives a number of common traits for the models it generates. I've ran into some use-cases where it would be helpful to be able to choose to derive additional things (e.g. https://beachape.com/frunk/frunk_core/labelled/trait.LabelledGeneric.html).

However, there are a potentially infinite number of different things we might wish to derive. As such, I wonder if we should add some mechanism whereby a user could provide a crate and a derive, which swagger-codegen could automatically add.

At the moment, a user could provide their own Cargo.mustache and models.mustache (c.f. #6414) and add the crates/derives themselves. Nonetheless, it might be useful to add proper support so that users don't need to maintain their own versions of the templates.

Swagger-codegen version

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Start by reviewing the rust-server templates Cargo.mustache and models.mustache, along with the approach described in issue #6414. Clarify how users would provide crates and derives and which configuration surface should control them. Done means generated Rust models can receive additional user-selected derives without maintaining custom template copies.