Restore possibility to define multiple URL per backend

[MPStudyly]

Hello everyone,

I just encountered an issue while helping to set up a spreed instance for a Nextcloud instance which is reachable via regular A (and AAAA) records, but also via a convenience (shorter) CNAME record.
When accessing the server via the A record, everything works fine and a connection to the signalling server can be established, but when connecting via the CNAME record it obviously fails, due to that URL missing in the signaling config. I noticed that before #28 was implemented, it was possible to specify multiple URLs within the "allowed" property. Since that change it is not possible to set multiple URLs anymore, which is required, when a single Nextcloud instance is reachable via multiple domains (regardless of DNS record types), such as in our scenario.
So my request is to re-allow specification of multiple URLs per backend, or, if that is not feasible, to allow communication between multiple backends when explicitly enabled via config toggle or by defining some sort of mapping (e.g. backend a can talk to b,c,d and vice versa, e can talk to f,g, etc.).

Best Regards
Michael

[fancycode]

This is currently not planned, but I'll happily accept pull-requests that implement multiple URLs per backend.