caching: fixing date header and response details (#11100)

Changing the HCM to not adjust the date header when serving (side-car) cached responses.
This includes adding a status flag for cached responses which propagates to access logs etc, as well as some test cleanup to support polling for multiple log entries.

Risk Level: Low (only affects pre-alpha cache filter)
Testing: new UT, enhanced IT
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ bb74a91ac413d81f4b1874ed6474df93e4ca2782

envoy/config/accesslog/v3/accesslog.proto

@@ -241,6 +241,7 @@ message ResponseFlagFilter {
         in: "IH"
         in: "DPE"
         in: "UMSDR"
+        in: "RFCF"
       }
     }
   }];

envoy/config/accesslog/v4alpha/accesslog.proto

@@ -240,6 +240,7 @@ message ResponseFlagFilter {
         in: "IH"
         in: "DPE"
         in: "UMSDR"
+        in: "RFCF"
       }
     }
   }];

envoy/data/accesslog/v3/accesslog.proto

@@ -186,7 +186,7 @@ message AccessLogCommon {
 }
 
 // Flags indicating occurrences during request/response processing.
-// [#next-free-field: 21]
+// [#next-free-field: 22]
 message ResponseFlags {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.data.accesslog.v2.ResponseFlags";
@@ -266,6 +266,9 @@ message ResponseFlags {
 
   // Indicates there was a max stream duration reached on the upstream request.
   bool upstream_max_stream_duration_reached = 20;
+
+  // Indicates the response was served from a cache filter.
+  bool response_from_cache_filter = 21;
 }
 
 // Properties of a negotiated TLS connection.