Fixing use of OpenSSL 3 deprecated functions

Hi,

I'm using rust-openssl as one of the cryptographic backends in Sequoia and just recently we've got a report that we're using functions deprecated in OpenSSL 3:

• DSA_free
• DSA_new
• DSA_set0_key
• DSA_set0_pqg
• ECDSA_do_verify
• EC_KEY_free
• EC_KEY_new
• EC_KEY_set_group
• EC_KEY_set_public_key
• EVP_PKEY_assign
• RSA_free
• RSA_new
• RSA_set0_key

I've went through the migration guide and it seems they want to encourage the usage of EVP_PKEY family of functions and quite a bit of them are missing here (EVP_PKEY_CTX_new_from_{name,pkey}) as well as the associated machinery (OSSL_PARAM_BLD, OSSL_PARAM_BLD_push_BN, ...).

Now, before I file a big PR I'd like to verify if my approach is correct:

One way to solve this would be to just expose the missing features and mark them as ossl300 but AFAICT this crate takes extra effort to provide consistent interface regardless of which library is used underneath. I wonder if a better approach would be just to fix the implementation so that it uses new functions when ossl300 is defined and uses the old API otherwise.

An example of existing code:

impl Rsa<Public> {
    pub fn from_public_components(n: BigNum, e: BigNum) -> Result<Rsa<Public>, ErrorStack> {
        unsafe {
            let rsa = cvt_p(ffi::RSA_new())?;
            RSA_set0_key(rsa, n.as_ptr(), e.as_ptr(), ptr::null_mut());
            mem::forget((n, e));
            Ok(Rsa::from_ptr(rsa))
        }
    }

This uses deprecated RSA_set0_key function and I think it should be changed to use EVP_PKEY_CTX_new_from_name passing BigNums via OSSL_PARAM_BLD_push_BN and, at the end, extracting the Rsa object out of the PKey but only when ossl300 is defined.

Not sure if all functions can be adjusted like that but from my casual skim most of them are for creating cryptographic objects.

Please confirm if this is a good approach or if there is a better one I should explore instead.

Thank you for your time! 👋

Start with the Rsa::from_public_components example and audit the listed deprecated APIs, including DSA, ECDSA, EC_KEY, EVP_PKEY, and RSA functions, against the OpenSSL 3 migration guide. Determine whether the missing EVP_PKEY_CTX and OSSL_PARAM_BLD features and conditional ossl300 implementation can preserve the crate's consistent interface; done means the deprecated uses are addressed without breaking other supported OpenSSL versions.