Backup installed flatpaks

### Problem

While setting up a new computer it is always annoying to have to reinstall all the software you need. So it would be nice to have an easy way to back up which flatpaks I have installed and then have appcenter reinstall those for me.

### Proposal

Provide a way to create a backup file that contains information about which flatpaks I have installed. Then I can store that file on my cloud or usb stick and have appcenter reinstall all my flatpaks for me on a new system just by providing it with the backup file.

### Prior Art (Optional)

_No response_