backupccl: SHOW BACKUP reports inaccurate size_bytes and file_bytes for small tables in coalesced ranges

SHOW BACKUP currently reports an estimate for the size of each backed up table in logical and physical bytes. This estimate assumes that each table lives in its own range. As of 22.2 and earlier, this assumption would occasionally lead to inaccurate estimates, such as if the backup was taken just after a table had been created, but before the table was split into its own range, and we lived with this transient inaccuracy. In 23.1 however, coalesced ranges will be default on, which means that SHOW BACKUP size estimates for small tables (i.e. those coalesced into a range), will be consistently inaccurate. We should address this.

Jira issue: CRDB-23384

Epic CRDB-21944