repo-pull: add option to set the async update frequency

repo-pull: add option to set the async update frequency

When using Flatpak with GNOME Software, it is important to
show the progress of the download and install as close as
possible to the real progress.

However, OSTree forces the frequency to call the async
progress callback to 1 second, which causes an unpleasant
effect on the UI, specially when the download size is so
small that everything happens in less than 1 second.

Fix that by adding making OSTree read a custom 'update-frequency'
option and set the timeout source timeout to that. If
no custom frequency is passed, we assume the default 1
second timeout, maintaining the current behavior.

Closes: #725
Approved by: jlebon