enhancement: support proto resources for grpc-transcode workflows

Summary

Please triage whether ADC should support managing proto resources needed by grpc-transcode workflows.

Current behavior

APISIX itself supports proto resources for grpc-transcode through the Admin API, including proto_id references from route plugin configuration, but ADC does not appear to expose a dedicated resource model for managing those proto descriptors.

As a result, a grpc-transcode setup that works in APISIX cannot currently be expressed cleanly end-to-end through ADC.

Use cases affected

• configuring grpc-transcode end-to-end through ADC, including both the route/plugin configuration and the proto descriptor it depends on
• managing proto descriptors declaratively together with the rest of the gateway configuration, instead of falling back to a separate manual step

Code context

A quick scan of the ADC schema and SDK types did not show a dedicated proto resource model for this workflow.