This predicate allows dual-stack RSA and ECDSA servers (for example),
and is an alternative for using the
certificate_key_pairs/1
option. As of OpenSSL 1.0.2,
multiple certificate types with completely independent certificate
chains are supported. If a certificate of the same type is added
repeatedly to a context, the result is undefined. Currently, up to 12
additional certificates of different types are admissible.