How to use a custom ClientCertResolver?

I'd like to be able to use a custom ClientCertResolver from C code that uses rustls-ffi to perform TLS. I currently have a resolver that implements ClientCertResolver that works fine with rustls inside of Rust code, but I can't see any way to use that within C code via rustls-ffi. Am I missing something that would make this work?

I've thought a little bit about how this sort of option might be exposed, and it seems like it might be a little complicated to make a good API for this. My thoughts are:

1. expose enough symbols on the rust side from rustls-ffi that it can be included as a library in another rust project, where that project exports a function rustls_result mylibrary_rustls_client_config_builder_set_custom_resolver(*rustls_client_config_builder builder);
2. expose a struct rustls_client_cert_resolver on the rust side that lets you convert a dyn ClientCertResolver into a type that can be understood in C code from rustls-ffi, and can be used like this:

rustls_client_cert_resolver resolver = mylibrary_custom_cert_resolver();
rustls_client_config_builder_set_client_cert_resolver(resolver);

Any opinions on this? I'm happy to send a PR if there are any reasonable options.

Start by reviewing the rustls-ffi C API around rustls_client_config_builder and the Rust ClientCertResolver type. Compare the two proposed ways to expose a custom resolver, then define a safe, usable API whose completion criteria include invoking a custom resolver from C and documenting its usage.