perf: 支持slack通知和认证 (#12193)

* perf: 支持slack通知和认证

* perf: 生成迁移文件

* perf: 优化获取access_token逻辑

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Co-authored-by: jiangweidong <weidong.jiang@fit2cloud.com>

apps/notifications/backends/__init__.py

@@ -12,6 +12,7 @@ class BACKEND(models.TextChoices):
     DINGTALK = 'dingtalk', _('DingTalk')
     SITE_MSG = 'site_msg', _('Site message')
     FEISHU = 'feishu', _('FeiShu')
+    SLACK = 'slack', _('Slack')
     # SMS = 'sms', _('SMS')
 
     @property