Summary: Diff adds support for a backend endpoint that will accept a storage config UUID and will find all potential backup directories within the storage config. This is done by uploading a temp file .yba_backup_marker inside the backup_dir when uploading a YBA backup. Then we query for the backup_dir and finds the directory right before.

Test Plan: create continuous backup config with multiple backup dirs, then query endpoint and make sure both directories are returned

Reviewers: dshubin, vkumar

Reviewed By: dshubin, vkumar

Subscribers: svc_phabricator, yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D42349