Unified Backup: Refactor Database Backup

What does this MR do and why?

This is a full refactor on the database backup code with some extractions to the gitlab-backup-cli gem.

Part of #427337 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.