Add an additional certificate/key pair to SSL0, yielding SSL.
Certificate and Key are either strings or atoms that hold the
PEM-encoded certificate plus certificate chain and private key,
respectively. Using strings is preferred for security reasons.
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.