Add new tool to migrate from MiaB to fresh server

[ahmadtechnik]

This PR introduces a new script that simplifies the process of migrating a Mail-in-a-Box (MiaB) installation from one server to another.
It’s designed to help MiaB users perform smooth, safe, and structured migrations with minimal effort.

Key Features:

• Automates the transfer of all relevant files and mailboxes from the source server to a new target server
• Generates a temporary SSH key for secure file syncing, used only during the migration
• Uses rsync over SSH to efficiently copy data while allowing exclusions
• Automatically updates the mailinabox.conf file, replacing the old IP with the new server’s public IP

Documentation:

• All usage instructions and parameter details are documented in the migrate.md file
• You’ll also find inline documentation at the top of the migrate.sh script

This tool aims to make MiaB migrations more accessible for all users.