Our TrafficPolicy supports configuring extproc for envoy, we should also support configuring agentgateway extproc.

• Add the proto type for extproc in agentgateway https://github.com/agentgateway/agentgateway/blob/main/crates/agentgateway/proto/resource.proto#L532
• Translate the extproc policy from the proto to the internal type in agent_xds.
• Add extproc translation in the agentgateway traffic policy plugin in kgateway https://github.com/kgateway-dev/kgateway/blob/main/pkg/agentgateway/plugins/traffic_plugin.go