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

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:] https://github.com/envoyproxy/envoy/blob/8188e232a9e0b15111d30f4724cbc7bf77d3964a/source/extensions/filters/http/ext_authz/ext_authz.cc#L176-L181