Support dual RSA and ECDSA certificates

ISSUE MIGRATED FROM https://gitlab.com/aegir/hosting_https/issues/36 (see #2934484: Move back issue management to D.o)

See https://letsencrypt.org/docs/integration-guide#supported-key-algorithms and https://scotthelme.co.uk/hybrid-rsa-and-ecdsa-certificates-with-nginx/

It looks like Apache can support this the same way: https://blog.joelj.org/dual-rsaecdsa-certificates-in-apache-2-4/