nixos/postfix: Use better types for submissionOptions and submissionsOptions (#138205)

nixos/modules/services/mail/postfix.nix

···
       };
 
       submissionOptions = mkOption {
-        type = types.attrs;
+        type = with types; attrsOf str;
         default = {
           smtpd_tls_security_level = "encrypt";
           smtpd_sasl_auth_enable = "yes";
···
       };
 
       submissionsOptions = mkOption {
-        type = types.attrs;
+        type = with types; attrsOf str;
         default = {
           smtpd_sasl_auth_enable = "yes";
           smtpd_client_restrictions = "permit_sasl_authenticated,reject";