[docdb] PITR: Metadata restore from external backups

[bmatican]

Jira Link: DB-1487
As a follow-up to #8846, we should figure out a way to allow metadata (DDL) restores from external backups.

For in cluster PITR, we achieve this by also taking a snapshot of the sys_catalog master tablet. One option would be to also start doing this for the normal backup operations and store this in external storage, thus allowing us to, on restore, look at the existing metadata and use the current PITR logic.

[bmatican]

Marking this as lower pri.

With in-cluster PITR, you configure a retention window.

• Within that window, you can quickly restore DML and DDL changes.
• Outside that window, you would need to rely on backups sitting on external storage.

Operationally, for being able to restore before a set of DDL changes, we can accomplish this as follows

• before a DDL migration, take backup1
• perform the migration
• afterwards, take backup2

If you would need to go back to before the migration, restore from backup1. If you need to go back to after the migration, restore from backup2. cc @dataindataout