feat: Add email options

sp-modules/simple-nixos-mailserver/options.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ config, lib, ... }:
 {
   options.selfprivacy.modules.simple-nixos-mailserver = {
     enable =
@@ -34,4 +34,54 @@
         };
       };
   };
+
+  options.sp.email = with lib; mkOption {
+    type = types.attrsOf (
+      types.submodule (
+        { name, ... }: {
+          options = {
+            subdomain = mkOption {
+              type = with types; strMatching "[A-Za-z0-9][A-Za-z0-9\-]{0,61}[A-Za-z0-9]";
+              example = "myservice";
+              default = config.sp.domain;
+              description = "Email subdomain";
+            };
+
+            systemdTargets = mkOption {
+              type = with types; listOf strMatchig "[a-zA-Z0-9@%:_.\-]+[.](service|socket|device|mount|automount|swap|target|path|timer|scope|slice)";
+              default = [];
+              example = [ "generate-mastodon-email-password.service" ];
+              description = "Systemd target which generates password file.";
+            };
+
+            sendOnly = mkOption {
+              type = with types; bool;
+              default = true;
+              example = "false";
+              description = "Specifies if the account should be a send-only account. Emails sent to send-only accounts will be rejected.";
+            };
+
+            hashedPasswordFile = mkOption {
+              type = with types; str;
+              example = "/run/keys/mastodon/email_password";
+              description = "Path where a file containing password hash located.";
+            };
+          };
+        }
+      )
+    );
+
+    description = ''
+      Don't use this option to create regular users!!!
+
+      This option gives modules possibility to create mailboxes to send emails, e.g. notifications or reminders.
+    '';
+
+    example = {
+      "noreply@mastodon.example.tld" = {
+        hashedPasswordFile = "/run/keys/mastodon/email_password";
+        systemdTargets = [ "generate-mastodon-email-password.service" ];
+      };
+    };
+  };
 }