Implement FETCH Control Messages

[mcintyrehh]

As a first step to building a working FETCH implementation, we should add the FETCH control message in moq-js.

The goal of this issue is to add everything to encode/decode the payload of a FETCH message, but not actually wire anything up.

This will unblock development in the backend where we can implement the same and ideally serve mocked responses to again unblock Playback/UI development.

I have a WIP of adding SUBSCRIBE_NAMESPACE control messages here that we can use for inspiration: mcintyrehh/moq-js@draft-06...mcintyrehh:moq-js:draft-06-part2-electric-boogaloo. This doesn't cover everything, but it should be a good start.

https://www.ietf.org/archive/id/draft-ietf-moq-transport-07.html#message-fetch

FETCH Message {
  Type (i) = 0x16,
  Length (i),
  Subscribe ID (i),
  Track Namespace (tuple),
  Track Name Length (i),
  Track Name (..),
  Subscriber Priority (8),
  Group Order (8),
  StartGroup (i),
  StartObject (i),
  EndGroup (i),
  EndObject (i),
  Number of Parameters (i),
  Parameters (..) ...
}

[mcintyrehh]

FYI @JoaquinBCh, Mike brought up a good point in the moq-rs issue, we should bump the version to draft-07 as well

[nicolaslevy]

Note from our meeting, @JoaquinBCh has started working on the FETCH messages