Description:

Describe the desired behavior, what scenario it enables and how it
would be used.

The API already exists, and the IR translation should already exist, but the code is missing in the translator layer
https://github.com/envoyproxy/gateway/blob/main/internal/xds/translator/extproc.go even when the ext proc filter supports it https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/core/v3/grpc_service.proto#envoy-v3-api-msg-config-core-v3-grpcservice

Relates to #6914

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Any extra documentation required to understand the issue.