draft-07: Implement FETCH passthrough for moq-relay-ietf

Initial wire format encoding/decoding for FETCH-related messages is done, but we don't yet handle them correctly.

Always forwarding upstream would be a valid implementation choice that doesn't require full cache implementation.

Start by tracing the existing FETCH-related wire-format encoding and decoding, then follow the moq-relay-ietf path where those messages are handled. Implement and verify the stated passthrough behavior by forwarding FETCH messages upstream without requiring a full cache implementation.

Written by the indexing model from the issue text.