Backups: Two units creating a backup at the same time could overwrite the backup file

Backups can be created on all units of a Valkey deployment (functionality added in #59). If the `create-backup` action is run at the exact same time on multiple units, they will create a backup with the same filename / backup-id, and the file uploaded to S3 storage might be corrupt/overwritten.

For details or reproducability see the review comment: https://github.com/canonical/valkey-operator/pull/59#pullrequestreview-4577257640

Start with the `create-backup` action and the reproducibility details in the review comment on pull request #59. Run the action concurrently on multiple deployment units and inspect the S3 backup objects and identifiers. Done means simultaneous backups no longer corrupt or overwrite one another.

Written by the indexing model from the issue text.