refactor: 邮件支持直接发送内容

agt-module-system/agt-module-system-api/src/main/java/org/agt/module/system/api/mail/dto/MailSendSingleToUserReqDTO.java

@@ -27,4 +27,7 @@ public class MailSendSingleToUserReqDTO {
 
     @Schema(description = "邮件模板参数列表")
     private List<Map<String, Object>> templateParamsList;
+
+    @Schema(description = "邮件内容")
+    private String content;
 }