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>

feature/routecheck/routecheck.go

@@ -19,6 +19,7 @@ import (
 	"sync"
 
 	"tailscale.com/ipn/ipnext"
+	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/net/routecheck"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
@@ -36,6 +37,8 @@ func init() {
 			backend: b,
 		}, nil
 	})
+
+	ipnlocal.HookRouteCheckReport.Set(routeCheckReport)
 }
 
 // Extension implements the [ipnext.Extension] interface.
@@ -162,3 +165,15 @@ func (e *Extension) reconcileLoop() {
 		}
 	}
 }
+
+func routeCheckReport(b *ipnlocal.LocalBackend) ipnlocal.RouteCheckReport {
+	c := ClientFor(b)
+	if c == nil {
+		return nil
+	}
+	r := c.Report()
+	if r == nil {
+		return nil
+	}
+	return r
+}