Extract file backup logic out into new class

What does this Merge Request do and why?

This MR extracts out the file backup logic into a new GDK::Backup class. This will prove useful for !1669 (closed).

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1114 (closed)