extensionSystem: Add install-from-HTTP capability

This adds a new DBus method: InstallExtensionRemote(uuid : s, url : s)

Pass it the UUID of an extension and the URL of a manifest file: the same as a
metadata.json, but with a special key, '__installer', which is an HTTP location
that points to an zip file containing the extension. The Shell will download
and use it to install the extension. In the future, the manifest file may be
used to automatically detect and install updates.

https://bugzilla.gnome.org/show_bug.cgi?id=654770