feat(devops): Add support for if-modified-since header for Feature Flags public endpoint

ref: #673 #663

app/core/exceptions.py

@@ -28,3 +28,14 @@ class APIError(
     default_detail = 'An unknown ERROR occured'
 
     default_code = 'unknown_error'
+
+
+class NotModified(
+    exceptions.APIException
+):
+
+    status_code = status.HTTP_304_NOT_MODIFIED
+
+    default_detail = ''
+
+    default_code = 'not_modified'