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

[englishm]

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:

• draft-07: Fetch plumbing and test utility #61
• draft-07: Error on FETCH in moq-relay-ietf #58
• draft-07: Implement FETCH passthrough for moq-relay-ietf #57
• Implement caching? #59