Restrict the use of keep annotations to full backups only

[bleidens]

An incremental backup with the keep annotation that has its parent deleted would become an orphan backup and therefore useless for recovery. To avoid this, we should restrict the use of keep annotations to full backups only. This adds a new check prior to starting the annotation process to make sure we do not allow keeping a backup if it has a parent backup, raising an error if so.

References: BAR-184

[bleidens]

I decided to add this check to the cli.py file inside the keep command since it also contains other checks and it seems better to make this check before starting an annotation. I also thought about adding this to annotations.py, inside KeepManagerMixin.keep_backup, but I need suggestions about the best place to make this check.

I will also still make updates for the case in which a parent backup exists but is empty, since this would pass the check in the current implementation, adding the keep annotation to the child backup.

[bleidens]

I just finished updating the unit tests and changed the check condition to only consider the existence of backup_info.parent_backup_id, no longer using the get_parent_backup_info method and avoiding passing the check in the case of empty parent backups.