Key agreement via SecKeyCopyKeyExchangeResult()

[crashdump]

Added SecKeyCopyKeyExchangeResult() to leverage Security.framework for ECDH-type key agreement.

[crashdump]

@kornelski, the code in this merge request works up to macOS 14, but it fails on 15+ due to the current implementation of SecKeyExt::from_data relying on SecKeyCreateFromData(), which is deprecated. Note that I do not believe key derivation to be possible today, and this MR shouldn't be breaking anything else.

I have successfully implemented key derivation on macOS 15+ by using the new method in my own code, however, I stopped short of making breaking changes in this project as I wasn't sure how you'd like to handle this.

Note that the new method to import public keys supports all the platforms, so it may warrant moving SecKeyExt outside the macos mod.

Let me know your thoughts, but this should be mergeable as-is - comments welcome.

Edit: In case someone stumbles on this while looking for a working solution to key exchange, this is the code in the host project.

let peer_public_key = unsafe {
    use core_foundation::base::TCFType;
    use core_foundation::data::CFData;
    use core_foundation::dictionary::CFDictionary;
    use core_foundation::error::CFError;
    use core_foundation::string::CFString;
    use security_framework_sys::item::{
        kSecAttrKeyClass, kSecAttrKeyClassPublic, kSecAttrKeyType,
        kSecAttrKeyTypeECSECPrimeRandom,
    };
    use std::ptr;

    let key_data = &CFData::from_buffer(&pub_key_raw);

    let dict = CFDictionary::from_CFType_pairs(&[
        (
            CFString::wrap_under_get_rule(kSecAttrKeyType),
            CFString::wrap_under_get_rule(kSecAttrKeyTypeECSECPrimeRandom).into_CFType(),
        ),
        (
            CFString::wrap_under_get_rule(kSecAttrKeyClass),
            CFString::wrap_under_get_rule(kSecAttrKeyClassPublic).into_CFType(),
        ),
    ]);

    let mut err = ptr::null_mut();
    let key = SecKeyCreateWithData(
        key_data.as_concrete_TypeRef(),
        dict.as_concrete_TypeRef(),
        &mut err,
    );
    if key.is_null() {
        Err(CFError::wrap_under_create_rule(err))
    } else {
        Ok(SecKey::wrap_under_create_rule(key))
    }
}?;

let shared_secret = self
    .key
    .key_exchange(
        Algorithm::ECDHKeyExchangeStandard,
        &peer_public_key,
        256,
        None,
    )
    .unwrap();