Add server_url to JSON storage file

[robalexdev]

I noticed that the libdns/acme-dns provider expects a server_url field to be defined in the JSON storage file. The acme-dns server doesn't set the server_url field in the JSON returned by the /register endpoint.

It would be helpful for the register API to include server_url to make these JSON blobs compatible. Currently, software like Caddy, which uses the libdns/acme-dns provider, will error out when this is missing (ServerURL cannot be empty). Other software defaults to https://auth.acme-dns.io which could accidentally leak information intended for a self-hosted installation to that endpoint.

Can acme-dns add the server's domain as a new server_url field? Including the server's URL directly with the username/password in the JSON storage file would make it more self-contained.