Update latest backup id search index to include 'msgBackup'

Summary:
This differential updatest backup table dynamodb schema. Without this change I was unable to get backup messages from dynamodb back even though I was able to put them
in the db. My understanding is that when fetching latest backup item we use index that previously wasn't aware of msg_backup field.

Test Plan: run terraform fmt and validate.

Reviewers: bartek, kamil

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11733