generator: support explicitly specifying PWA manifest URL
I wanted to add Gmail to the proprietary webapp list. GMail has a manifest and it is linked from the header of https://mail.google.com/ – but only if you are logged in. If you are not logged in, as is the case for pwa-metainfo-generator.py
, you are redirected to an https://accounts.google.com/ login page, which does not have a linked manifest.