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)