uci-git-backup: add configuration backup package

[mrangger]

Summary

• add a new uci-git-backup package under utils/ for automatic UCI configuration backups to a remote Git repository
• include the runtime backup, connection-test, history, and restore helper scripts together with the init script and default UCI config
• support both HTTPS token authentication and SSH key authentication for unattended router backups

[stangri]

@mrangger thanks for this Mathias. I wonder if you could address a few questions I have after looking at this?

1. Why not use/port etckeeper?
2. What's the combined size of dependencies of this package?
3. Why not initialize git in the /etc/config directly instead of using an additional git dir?
4. If you're not initializing git in /etc/config directly, and are using an additional git dir, why not also add files from /etc/sysupgrade.conf?
5. Maybe you'd want to use service_data() function to populate the ubus with the git command results/errors.

Thanks again!