Feature Request: Allow for backups to be created that are safe to upgrade MySQL with

[dbussink]

Feature Description

In vtbackup it's possible to use the -restart_before_backup flag to force a restart of MySQL with innodb_fast_shutdown=0. This way the backup that is created is safe to also use for upgrading MySQL. When upgrading MySQL, it's necessary to not use fast shutdown as the new version might have a different on disk format.

In Backup / BackupShard commands though through vtctld, it's not possible to specify a similar flag. This means that backups created through that method couldn't be safely used for upgrades.

Request here is to have a flag available for those backup methods as well so it's possible to create a safe to upgrade backup.

Use Case(s)

Useful when upgrading MySQL in a Vitess cluster, especially when doing it in an automated fashion for a large number of databases.