[feature request] support for custom derive

There are some quality-of-life derives that would be really nice to have for C++ types. e.g. `#[derive(TryFromPrimitive)]` from `num_enum`.

Maybe `generate!` could take a list of derives that user want to add?

Start by tracing the `generate!` entry point and how it produces Rust types from C++ headers. Compare the requested `TryFromPrimitive` derive with the existing generated-type flow and determine the supported API for passing custom derives. Done means a documented, tested way to apply user-specified derives.

Written by the indexing model from the issue text.