DRAFT: Implement gnome-extensions:// protocol.

Add a simple protocol to install extensions.

Currently supports gnome-extensions://install?uuid=name@name.com

This is meant to address systems where extensions.gnome.org does not have access to the native integration. This is common on GNOME OS images, with systems where Firefox and/or Chrome are containerized, for users who choose to only use Epiphany, and in any case where users don't want to install the integration.

In future versions (follow ups) we can potentially add feedback in-app for installation failures.