ipn: add fmt.Stringer support to NotifyWatchOpt (#20072)

This patch adds support for the fmt.Stringer interface to the
ipn.NotifyWatchOpt enum. This is useful when debugging these bitmasks.

For example:

	fmt.Printf("%s", ipn.NotifyPeerChanges | ipn.NotifyNoNetMap)
	// Output: (ipn.NotifyPeerChanges | ipn.NotifyNoNetMap)

Fixes #20066

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

client/local/local.go

@@ -1365,8 +1365,12 @@ func (lc *Client) StreamDebugCapture(ctx context.Context) (io.ReadCloser, error)
 //
 // A default set of ipn.Notify messages are returned but the set can be modified by mask.
 func (lc *Client) WatchIPNBus(ctx context.Context, mask ipn.NotifyWatchOpt) (*IPNBusWatcher, error) {
+	m, err := mask.MarshalText()
+	if err != nil {
+		return nil, err
+	}
 	req, err := http.NewRequestWithContext(ctx, "GET",
-		"http://"+apitype.LocalAPIHost+"/localapi/v0/watch-ipn-bus?mask="+fmt.Sprint(mask),
+		"http://"+apitype.LocalAPIHost+"/localapi/v0/watch-ipn-bus?mask="+string(m),
 		nil)
 	if err != nil {
 		return nil, err