control/controlclient: handle 429 responses during node registration

If we get a 429 response during node registration, use the `Retry-After`
header for backoff instead of the regular exponential backoff.

The rate limiter error is propagated to the user, just like other
registration errors are, e.g.

```
$ tailscale up
backend error: node registration rate limited; will retry after 57s
exit status 1
```

Updates tailscale/corp#39533

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

control/controlclient/auto.go

@@ -356,7 +356,15 @@ func (c *Auto) authRoutine() {
 		if err != nil {
 			c.direct.health.SetAuthRoutineInError(err)
 			report(err, f)
-			bo.BackOff(ctx, err)
+			if rle, ok := errors.AsType[*rateLimitError](err); ok {
+				c.logf("authRoutine: %s", rle)
+				select {
+				case <-ctx.Done():
+				case <-time.After(rle.retryAfter):
+				}
+			} else {
+				bo.BackOff(ctx, err)
+			}
 			continue
 		}
 		if url != "" {