Introduce 'msgBackup' field to 'OrderedBackupItem' and 'LatestBackupIDResponse'

Summary: This differential introduces backup message field to OrderedBackupItem so that backup message can be fetched from dynamodb.

Test Plan: For now ensure that backup functionality remains unbroken. Very next diff will enable testing with simple button click.

Reviewers: kamil, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11734