Introduce new JSI call to retrieve backup message for latest siwe backup
Summary: This differential introduces new JSI call to fetch latest backup id and associated backup message for SIWE user.
Test Plan:
- Log in as SIWE user.
- In use-backu-client add call to this function after awaiting createNewBackup. Log the result.
- Ensure that the result logged is the same as backup message in dynamodb web viewer.
Reviewers: kamil, bartek
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D11735