Draft: feat: var BACKUP_SERVER to tag server as backup

[pini-gh]

Related to nginx-proxy/acme-companion#1062

[SchoNie]

Would it be an idea to use a more generic variable to set parameters of a server?
For example in my setup I need to specify weight=5;
And somebody in #2403 needed fail_timeout=30s;

Something like SERVER_PARAMETERS which can contain any parameter like backup, weight, fail_timeout (or a combination) from https://nginx.org/en/docs/http/ngx_http_upstream_module.html#server