add bulk actions support

fastapi_admin/schemas.py

@@ -1,3 +1,5 @@
+from typing import List
+
 from fastapi import Body
 from pydantic import BaseModel
 
@@ -5,3 +7,7 @@ from pydantic import BaseModel
 class LoginIn(BaseModel):
     username: str = Body(..., example='long2ice')
     password: str = Body(..., example='123456')
+
+
+class BulkIn(BaseModel):
+    pk_list: List = Body(..., example=[1, 2, 3])