xds: enable the `is_optional` field for HttpFilter (#16119)

Enable `is_optional` field for `HttpFilter`. The default value is `false`, it will keep the same behavior with the current implementation. The envoy will reject the unsupported HTTP filter, also will reject the unsupported HTTP filter in typed per filter config. When the value is `true`, the unsupported HTTP filter will be ignored by the envoy, also will be ignored by typed per filter config, with a warning log.

Risk Level: low
Testing: unit tests and integration tests are added
Docs Changes: API doc is added
Release Notes: added as new feature
Fixes #15770
Fixes #15025

Signed-off-by: He Jie Xu <hejie.xu@intel.com>

Mirrored from https://github.com/envoyproxy/envoy @ ea4cadcd0e33da0c0e8272131866874b0ea377fb

envoy/extensions/filters/network/http_connection_manager/v3/http_connection_manager.proto

@@ -979,7 +979,7 @@ message HttpFilter {
   // If true, clients that do not support this filter may ignore the
   // filter but otherwise accept the config.
   // Otherwise, clients that do not support this filter must reject the config.
-  // [#not-implemented-hide:]
+  // This is also same with typed per filter config.
   bool is_optional = 6;
 }
 

envoy/extensions/filters/network/http_connection_manager/v4alpha/http_connection_manager.proto

@@ -964,7 +964,7 @@ message HttpFilter {
   // If true, clients that do not support this filter may ignore the
   // filter but otherwise accept the config.
   // Otherwise, clients that do not support this filter must reject the config.
-  // [#not-implemented-hide:]
+  // This is also same with typed per filter config.
   bool is_optional = 6;
 }