Add LatestURL key to sysroot latest command

It returns the remote URL from where the sysroot can be downloaded
so that application integrating the tool can use their own
download manager.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5207