draft-07: Serve Fetch responses from local cache in moq-relay-ietf

We have basic encoding/decoding for Fetch control messages already, but don't yet properly respond to them when we receive them in `moq-relay-ietf`.

Fully implementing Fetch requires us to have a local cache and to also correctly determine when and how we need to go upstream to fill that cache with additional objects.

We should probably work towards serving Fetch response by first tackling these other issues:
- https://github.com/englishm/moq-rs/issues/61
- https://github.com/englishm/moq-rs/issues/58
- https://github.com/englishm/moq-rs/issues/57
- https://github.com/englishm/moq-rs/issues/59

Start by reviewing the existing Fetch control-message encoding and decoding in moq-relay-ietf, then read issues 61, 58, 57, and 59 for the prerequisite cache and upstream-fetch work. Done means the relay can maintain a local cache, fetch additional objects upstream when needed, and serve Fetch responses from that cache.

Written by the indexing model from the issue text.