feature/conn25,ipn/ipnext,ipn/ipnlocal: add ExtraRouterConfigRoutes hook

conn25 needs to add routes to the operating system to direct handling
of the addresses in the magic IP range to the tailscale0 TUN and
tailscaled.

The way we do this for exit nodes and VIP services is that we add routes
to the Routes field of router.Config, and then the config is passed to
the WireGuard engine Reconfig.

conn25 is implemented as an ipnext.Extension and so this commit adds a
hook to ipnext.Hooks to allow any extension to provide routes to the
config. The hook if provided is called in routerConfigLocked, similarly
to exit nodes and VIP services.

Fixes tailscale/corp#38123

Signed-off-by: Fran Bull <fran@tailscale.com>

feature/conn25/conn25.go

@@ -33,6 +33,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/appctype"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/views"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
@@ -123,12 +124,18 @@ func (e *extension) Init(host ipnext.Host) error {
 	}
 	e.host = host
 	host.Hooks().OnSelfChange.Add(e.onSelfChange)
+	host.Hooks().ExtraRouterConfigRoutes.Set(e.getMagicRange)
 	ctx, cancel := context.WithCancelCause(context.Background())
 	e.ctxCancel = cancel
 	go e.sendLoop(ctx)
 	return nil
 }
 
+func (e *extension) getMagicRange() views.Slice[netip.Prefix] {
+	cfg := e.conn25.client.getConfig()
+	return views.SliceOf(cfg.magicIPSet.Prefixes())
+}
+
 // Shutdown implements [ipnlocal.Extension].
 func (e *extension) Shutdown() error {
 	if e.ctxCancel != nil {
@@ -498,6 +505,12 @@ type client struct {
 	config        config
 }
 
+func (c *client) getConfig() config {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.config
+}
+
 // ClientTransitIPForMagicIP is part of the implementation of the IPMapper interface for dataflows lookups.
 func (c *client) ClientTransitIPForMagicIP(magicIP netip.Addr) (netip.Addr, error) {
 	c.mu.Lock()
@@ -673,7 +686,7 @@ func makePeerAPIReq(ctx context.Context, httpClient *http.Client, urlBase string
 }
 
 func (e *extension) sendAddressAssignment(ctx context.Context, as addrs) error {
-	app, ok := e.conn25.client.config.appsByName[as.app]
+	app, ok := e.conn25.client.getConfig().appsByName[as.app]
 	if !ok {
 		e.conn25.client.logf("App not found for app: %s (domain: %s)", as.app, as.domain)
 		return errors.New("app not found")