When a client selects a particular exit node, Control may use that as
a signal for deciding other routes.
This patch causes the client to report whenever the current exit node
changes, through tailcfg.Hostinfo.ExitNodeID. It relies on a properly
set ipn.Prefs.ExitNodeID, which should already be resolved by
`tailscale set`.
Updates tailscale/corp#30536
Signed-off-by: Simon Law <sfllaw@tailscale.com>