Implement FETCH Control Message

[mcintyrehh]

As a first step to building a working FETCH implementation, we should add the FETCH control message. The goal of this issue is to add everything to encode/decode the payload of a FETCH message and serve mocked responses as a means of unblocking development on the playback side of the house.

This is related to englishm/moq-js#35.

I have a WIP of adding MAX_SUBSCRIBE_ID control messages here that we can use for inspiration: https://github.com/englishm/moq-rs/compare/me/draft-06...mcintyrehh:moq-rs:draft-06-ext?expand=1. 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 (..) ...
}

[englishm]

Even if we do a very minimal PoC implementation of FETCH on a branch at first, we should bump the version number to draft-07 to enable testing against other implementations.

[mcintyrehh]

Yeah that is a great call, should we create a new me/draft-07 branch to merge these changes into?