route.proto: add is_optional field to ClusterSpecifierPlugin (#20301)
route.proto: add is_optional field to ClusterSpecifierPlugin (#20301)

When deploying a new cluster specifier plugin, it is often necessary to add it to the configuration before all clients can be updated to support it, with routing rules configured to prevent clients without support from selecting any routes referencing the plugin.  This field will allow those clients to suppress the default behavior of NACKing any resource containing the unknown plugin.

Risk Level: None
Testing: None
Docs Changes: None
Release Notes: None
Platform Specific Features: None

Signed-off-by: Doug Fawley <b80273c3e415c4baf79e8c71722f0957bec37c05@google.com>
envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy