Add variable for extra domains to get LE certs for

I felt that adding another variable was probably going to be the easiest way to do this. I may end up adding another variable to enable this feature, for consistency with some of the other things.

group_vars/matrix_servers

@@ -1025,6 +1025,8 @@ matrix_ssl_domains_to_obtain_certificates_for: |
     ([matrix_server_fqn_jitsi] if matrix_jitsi_enabled else [])
     +
     ([matrix_domain] if matrix_nginx_proxy_base_domain_serving_enabled else [])
+    +
+    matrix_ssl_additional_domains_to_obtain_certificates_for
   }}
 
 matrix_ssl_architecture: "{{