mailsystem: Add option extraSettingsFile to allow partial encryption of configuration

mailsystem/default.nix

@@ -206,6 +206,18 @@ in {
       default = {};
     };
 
+    extraSettingsFile = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
+      description = ''
+        YAML file to merge into the mailsystem configuration at runtime.
+        This can be used to store secrets, and, more importantly, keep your email
+        addresses out of the hands of spammers. This `extraSettingsFile` currently
+        supports `domains`, `accounts` and `virtualAliases` which can be defined in
+        the same manner as they can be via nix.
+      '';
+      default = null;
+    };
+
     certificateScheme = lib.mkOption {
       type = lib.types.enum ["acme" "selfsigned"];
       default = "acme";