Allow ACME to request non-ejabberd hosted subdomains

The current implementation just uses ACME to get certs for: domain.tld, upload.domain.tld, conference.domain.tld, proxy.domain.tld, pubsubdomain.tld which, I guess, are extracted from the currently hosted ones by ejabberd.

ejabberd, at least in my case, is not just a single daemon for everything on this domain, I might have others too in need of certs, like web.domain.tld (hosting Converse.js on nginx) or anything else really.

So I am forced to add some other ACME tool to generate some certs that might not share the same domain key, so basically different certs with different SANs and expiration dates, on the same base domain.

Now, I do see the point of keeping it strictly related to ejabberd, but the whole thing is rather odd:

• If I use dehydrated for web.domain.tld why should I bother with the included ACME? I can use this for the ejabberd domains too, right?
• If ejabberd is getting certs for its hosted domains only, why does it generate a cert for upload.domain.tld even if that is NOT hosted by itself (put_url points to a nginx instance, and with external_secret via ngx_http_upload) ? nginx should now use one cert for upload. and another for web.

Maybe add an option additional_domains: where the admin can, besides whatever ejabberd is setup, add other domains.

Right now this feels artificially limited for some reason.