Support set a proxy manually for flatpak operation

[BLumia]

This is a feature request.

In some region, it can be pretty slow to access flathub for updating flatpak packages, but accessing Steam's server won't be an issue. In such case, allowing setting a proxy only for flatpak operation instead of setting a system-level proxy could be handy.

The flatpak executable will use the proxy specified in https_proxy environment variable to access the flatpak remote repo. So to implement such feature, we can add a setting option to allow user enter a proxy url (e.g. http://127.0.0.1:11451), check if the option is empty, and set the https_proxy environment variable before running flatpak commands if it's not empty could be ok.

I can help implementing this if it's acceptable but it might take some time to learn about how the decky thing works before I can start hacking. It could be great if you can implement it if possible 😂

Let me know if you need any additional information. Thanks!