ipn/ipnlocal: add IPN Bus NotifyRateLimit watch bit NotifyRateLimit

Limit spamming GUIs with boring updates to once in 3 seconds, unless
the notification is relatively interesting and the GUI should update
immediately.

This is basically @barnstar's #14119 but with the logic moved to be
per-watch-session (since the bit is per session), rather than
globally. And this distinguishes notable Notify messages (such as
state changes) and makes them send immediately.

Updates tailscale/corp#24553

Change-Id: I79cac52cce85280ce351e65e76ea11e107b00b49
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -2780,20 +2780,17 @@ func (b *LocalBackend) WatchNotificationsAs(ctx context.Context, actor ipnauth.A
 		go b.pollRequestEngineStatus(ctx)
 	}
 
-	// TODO(marwan-at-work): check err
 	// TODO(marwan-at-work): streaming background logs?
 	defer b.DeleteForegroundSession(sessionID)
 
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case n := <-ch:
-			if !fn(n) {
-				return
-			}
-		}
+	sender := &rateLimitingBusSender{fn: fn}
+	defer sender.close()
+
+	if mask&ipn.NotifyRateLimit != 0 {
+		sender.interval = 3 * time.Second
 	}
+
+	sender.Run(ctx, ch)
 }
 
 // pollRequestEngineStatus calls b.e.RequestStatus every 2 seconds until ctx