Add proxy settings

[thmsklngr]

Pre-submit checks

• [X] I checked for similar issues beforehand, but could not find any. I could not add my proposal to any existing issue.
• [X] I am going to take the time to to fill in all the required details. I know that the bug report will be dismissed otherwise.

Feature description

I use AppImageLauncher behind a firewall and have to use a proxy to go outside. Currently there's no option available to configure an optional proxy+port for appimagelauncherd.

Proposed solution

Add the possibility to add lines like below in ~/.config/appimagelauncher.cfg:

[appimagelauncherd]
http_proxy = your.proxy.here:8080
https_proxy = your.proxy.here:8080

But they should be optional.

Alternative solutions you considered

No response

Additional context

No response