Introduce msg_backup field to BackupItem
Summary: This differential introduces new field - msg_backup to BackupItem so that backup data can be persisted in dynamodb with respective backup message.
Test Plan:
- Locally trigger backup upload from password user.
- Ensure that it works and using web inspector of dynamodb see that there is a new column with undefined value.
Reviewers: bartek, kamil
Reviewed By: bartek, kamil
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D11713