ipn/ipnlocal: use routecheck reports to make exit node suggestions

Now that the routecheck subsystem is continuously collecting
reachability reports in the background, we can add a hook to
LocalBackend for fetching its report. That allows
suggestExitNodeUsingTrafficSteering to consult that report when
disqualifying candidates, instead of blocking on an immediate probe.

Exit node suggestions will only consult the report when the
`client-side-reachability` and `client-side-reachability-routecheck`
node attributes are both set on the current node.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tsnet/depaware.txt

@@ -202,7 +202,8 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
         tailscale.com/net/portmapper                                 from tailscale.com/feature/portmapper
         tailscale.com/net/portmapper/portmappertype                  from tailscale.com/net/netcheck+
         tailscale.com/net/proxymux                                   from tailscale.com/tsnet
-        tailscale.com/net/routecheck                                 from tailscale.com/client/local
+        tailscale.com/net/routecheck                                 from tailscale.com/client/local+
+        tailscale.com/net/routecheck/peernode                        from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/net/sockopts                                   from tailscale.com/wgengine/magicsock
         tailscale.com/net/socks5                                     from tailscale.com/tsnet
         tailscale.com/net/sockstats                                  from tailscale.com/control/controlclient+