[backup-service][backup-client] update /latest endpoint to return both backupID and userID
Summary:
ENG-6145.
We need this to return userID too.
Renaming path and following types to make it more readable.
Depends on D13808
Test Plan: run commtest
Reviewers: varun, bartek
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D13809