Allow cancelling the downloading process for extra data

So far, the installation of external apps can only be cancelled
before flatpak starts downloading the extra data, as there's no
cancellable being passed to g_input_stream_read_async().

This fixes that problem, making it possible to cancel installs
from GNOME Software regardless of the installation stage.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f