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>
958bcda5bf