Add a new UsingCachedNetworkMap flag to the ipnstate.Status message.
Populate it as true when self has a network map that was loaded from cache,
vs. being directly provided by the control plane.
Updates #12639
Change-Id: I0fd5b1d6ec8df5cec5d4a3fa6b263293124780d1
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>