decompressor: add config flag to ignore no-transform (#19399)

This allows matching the more lenient behavior of other client-side
networking libraries such as OkHttp or URLSession, especially in cases
where the remote server is not under the client developer's control.

Risk Level: Low, defaults are unaffected, change is opt-in.
Testing: Added unit tests for existing no-transform behavior (there was none) and a new test validating the new configuration flag.
Docs Changes: Updated.
Release Notes: Added.

Signed-off-by: JP Simard <jp@jpsim.com>

Mirrored from https://github.com/envoyproxy/envoy @ 2afb9573ab1915ed57ac8d42203286b224ce95ae

envoy/extensions/filters/http/decompressor/v3/decompressor.proto

@@ -25,6 +25,9 @@ message Decompressor {
     // Runtime flag that controls whether the filter is enabled for decompression or not. If set to false, the
     // filter will operate as a pass-through filter. If the message is unspecified, the filter will be enabled.
     config.core.v3.RuntimeFeatureFlag enabled = 1;
+
+    // If set to true, will decompress response even if a *no-transform* cache control header is set.
+    bool ignore_no_transform_header = 2;
   }
 
   // Configuration for filter behavior on the request direction.