mail-server: add dmarcReporting.excludeDomains

The option `exclude_domains` for dmarc reporting in `rspamd`[1] allows
to configure a list of domains and/or eSLDs (external effective second level
domain) to be excluded from dmarc reports.

Helpful because e.g. dmarc reports to hotmail.com always fail for me
with the following undeliverable notification:

    The recipient's mailbox is full and can't accept messages now.

[1] https://www.rspamd.com/doc/modules/dmarc.html

default.nix

@@ -894,6 +894,14 @@ in
           The sender name for DMARC reports. Defaults to the organization name.
         '';
       };
+
+      excludeDomains = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        description = ''
+          List of domains or eSLDs to be excluded from DMARC reports.
+        '';
+      };
     };
 
     debug = mkOption {