Support for Multi-Destination Backups

[Theryston]

Description:
Allow gib backup to simultaneously write a single backup to multiple storage backends (e.g., one local disk + one S3 bucket) in the same run.

Why:
This removes the need to run multiple backup jobs and ensures redundancy without additional scripting. It's a frequently requested feature from users who want high resilience (local + cloud) or multiple cloud providers.

Proposed Syntax (example):

gib backup --storage local --storage s3

Expected Behavior:

• Upload chunks and snapshot metadata to all specified storages concurrently.
• Failures in one storage should not affect the others (unless explicitly required).
• Ideally, support resumable or retry logic per storage.