Add plugin system and notice plugin (#503)

* Update system notice to plugin

* Add plugin model alembic support

* update plugin conf

* add plugin route injection

* update plugin route inject

* fix and optimize plugin router inject

backend/main.py

@@ -1,20 +1,5 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
-from pathlib import Path
-
-import uvicorn
-
 from backend.core.registrar import register_app
 
 app = register_app()
-
-
-if __name__ == '__main__':
-    # 如果你喜欢在 IDE 中进行 DEBUG，main 启动方法会很有帮助
-    # 如果你喜欢通过 print 方式进行调试，建议使用 fastapi cli 方式启动服务
-    try:
-        config = uvicorn.Config(app=f'{Path(__file__).stem}:app', reload=True)
-        server = uvicorn.Server(config)
-        server.run()
-    except Exception as e:
-        raise e