Added the following helpers:
- generate_backup_id - generates IDs for new backups. C++ counterpart: link
- generate_blob_holder - generates holder string for blob service. C++ counterpart: link
- generate_random_string - random alphanumeric str of given length. C++ counterpart: link
- BackupItem::new - database entity convenience constructor
- handle_db_error - utility for mapping database errors into grpc status. It's almost copy-pasted from Identity service.
These are all needed for the CreateBackup handler.
Depends on D6196