Support ECDSA+RSA dual stack certificates

[github-k8n]

Most other programs support to specify two certificates for the same hostname (one RSA certificate and one ECDSA certificate) (nginx, exim, dovecot, prosody, ...)

Currently only a single certificate can be used for each host when using ejabberd.

This leads to having to decide between better performance (ECDSA cert) or better compatibility (RSA cert)

Please add an option to be able to specify multiple certificates/keys per hostname.