Release Summary:

Add a synchronous rust binding API for s2n_cert_validation_callback

Resolved issues:

resolves #5264

Description of changes:

s2n_cert_validation_callback is an unstable async callback that operates on the struct s2n_cert_validation_info. However, due to a lifetime issue (#5469) in our rust bindings, this PR only exposed the synchronous rust API for the cert validation callback.

Code changes breakdown:

1. Added a new module cert_validation.rs and gated behind feature = "unstable-crl".
2. Wrapped s2n_cert_validation_info in a Rust struct CertValidationInfo and implemented accept() and reject() methods corresponding to the C APIs.
3. Defined the trait CertValidationCallbackSync and added a config API set_cert_validation_callback_sync().
4. Tested the synchronous callback with different outcomes by calling accept() or reject().

Testing:

• Implemented CertValidationCallbackSync for a test struct SyncCallback
• Application specific context can be store on Connection via set_application_context()

I ran the unit test locally and confirmed it passed:

$ cargo test --features unstable-crl
...
test callbacks::cert_validation::tests::sync_cert_validation ... ok
...

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.