Feature Request: Add more flexibility backing up/restoring from different backup engines.

[rvrangel]

Feature Description

Today even though a vttablet can restore backups take from different backup engines, we can't change which backup engine we want to use unless we have tablets with different configuration in the same shard or changing it and restarting.

I would propose the following:

• adding a field on the Backup() call to allow to pass which backup engine to use, so users can have all the flags and define a default backup engine, but are still able to override which backup engine to use in the Backup() call
• adding a field on the RestoreFromBackup() call so backups from certain backup engines can be ignored when a tablet either starts for the first time or when received via its tabletamanger API`.

Use Case(s)

This will be interesting specially in combination with #16294 as it will allow for the particular scenario of being able to take both physical and logical backups, but always restoring physical backups by default. We would still be able to target a specific backup to be restored via and API call via its timestamp.

[shlomi-noach]

I like this idea.