Add file.list_backups execution function

This commit adds a function that will list the available backups for a
given path.
Add file.restore_backup, file.delete_backup

These functions allow you to restore a backup found with
file.list_backups, as well as remove unwanted backups.
Document backup listing/restoring/deleting
add information to backup restoration docs
Merge pull request #6376 from terminalmage/issue6322

Add functions to manage file backups