msmtp: add extraConfig account option

This patch allow to define custom msmtp options per email account. For
example: to change the "auth" method from "on" to "login", add
`msmtp.extraConfig.auth="login"`.

modules/programs/msmtp-accounts.nix

@@ -21,5 +21,16 @@ with lib;
         primary account will be used.
       '';
     };
+
+    extraConfig = mkOption {
+      type = types.attrsOf types.str;
+      default = { };
+      example = { auth = "login"; };
+      description = ''
+        Extra configuration options to add to <filename>~/.msmtprc</filename>.
+        See <link xlink:href="https://marlam.de/msmtp/msmtprc.txt"/> for
+        examples.
+      '';
+    };
   };
 }

modules/programs/msmtp.nix

@@ -31,6 +31,7 @@ let
           # msmtp requires the password to finish with a newline.
           passwordeval = ''${pkgs.bash}/bin/bash -c "${toString passwordCommand}; echo"'';
         }
+        // msmtp.extraConfig
       )
       ++ optional primary "\naccount default : ${name}"
     );