feat(bindings): expose cert validation callback

[CarolYeh910]

Release Summary:

Add rust binding 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. To expose its rust binding, I utilized the existing AsyncCallback struct and made the following changes:

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 CertValidationCallback and added a config API set_cert_validation_callback().
4. Tested the callback in synchronous and async use cases.

Testing:

• Implemented CertValidationCallback for both SyncCallback and AsyncCallback.
• AsyncCallback returns a struct AsyncFuture that implements ConnectionFuture and will execute the async validation. Application specific context can be store on Connection via set_application_context().

I ran the tests locally and confirmed they both passed:

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