Add X509_get0_authority_key_id() function

Add X509_get0_authority_key_id() function

This function makes it easier to retrieve a reference to the
authority key identifier (akid->keyid) inside a certificate.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5271)

(cherry picked from commit b383aa2081467e8d49c3362d295da7bd5cb4e1d8)