Support http_429 in *_cache_use_stale
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Use case: we use nginx with fastcgi_cache to cache responses from a backend application. Clients can request nginx-cached endpoints without limits, but the backend performs rate limiting to protect itself by returning a HTTP 429 if the limit is exceeded.
It would be very useful to allow serving a stale cache entry when the upstream returns a 429, since the client would probably prefer a stale reply over no reply at all.