Add an extension point so third party plugins can be installed

[piperswe]

Documentation on Flatpak extensions: https://docs.flatpak.org/en/latest/extension.html

[Pixelnarium]

Last time I tried it was not possible since plugins needed dependencies in /app/lib and every plugin goes to /app/lib/purple-2, the same directory that Pidgin uses. And I was unable to merge that directory since it belongs to the main application.

But if you have an idea how to do it, feel free to provide an example.