backup: add marker file in backups for TTL detection

Currently, the manifest file indicates that a backup is complete and restorable. However, taking into consideration bucket TTL policies, this is not necessarily true. The manifest file is the last file written in a backup, which means that the data files will expire before the manifest expires, so it is entirely possible that a backup contains a manifest but is not restorable.

We should write a file at the start of a backup to ensure it expires before all other files and when checking if a backup is restorable, we look for the existence of both the manifest and the file.

Context: https://cockroachlabs.slack.com/archives/C03JCUUSCD6/p1753465882995189

Epic: CRDB-52076

Jira issue: CRDB-53010