[services] Backup - use generateHolder for new backup

Summary:
Depends on D4030

https://linear.app/comm/issue/ENG-1128/implement-generating-holders

Test Plan:
Run the blob service
Run the backup service
Use grpc-playground to make a call creating a new backup

See the generated holder in the database or log it out

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4031