add a way to configure the `proxy_cache_*` [1] directive for external-auth.
The user-defined cache_key may contain sensitive information
(e.g. Authorization header).
We want to store *only* a hash of that key, not the key itself on disk.
[1] http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_key
Signed-off-by: Moritz Johner <beller.moritz@googlemail.com>
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