Ext-Authz: Perform clear route cache based on specific headers.

[VirajSalaka]

Description:
The concern here is as same as mentioned in #12195. In my usecase, I would like to make the routing decision based on a specific header (using cluster-header property) for some routes only. And those headers would be injected from the external Auth filter. To make this work, we need to set the clear_route_cache property to true at the moment. As per the current design, the cache clearing process will only be triggered if we modify the headers inside the ext_authz filter.

But the concern here is we may require to populate various headers within the ext_authz filter and for all these headers this cache clearing process will happen even though it is not required. Hence I would like to suggest adding another separate field to specifically mention those headers as an array of strings. (Something like clear_route_cache_on_headers)

Based on #12195, I guess there are some other users who would be interested in the same feature. I would like to know your opinion on this.

[optional Relevant Links:]

envoy/source/extensions/filters/http/ext_authz/ext_authz.cc

Lines 176 to 181 in 8188e23

	if (config_->clearRouteCache() &&
	(!response->headers_to_set.empty() || !response->headers_to_append.empty() ||
	!response->headers_to_remove.empty())) {
	ENVOY_STREAM_LOG(debug, "ext_authz is clearing route cache", *callbacks_);
	callbacks_->clearRouteCache();
	}

[esmet]

I'll take a look at this. @VirajSalaka would you say that clear_route_cache_on_headers is an allow-list of headers that are set, appended, or removed only when clear_route_cache is set to true?

[esmet]

/assign