Feature request: Remove NATS completely

[neekolas]

Is your feature request related to a problem?

We get very little value from Nats, and it makes our streaming setup more complicated. We should just get rid of it altogether.

Describe the solution to the problem

1. Migrate MLS to use the same subscription dispatcher that api/v1 is using
2. Remove dependency on nats from the subscription dispatcher and instead rely on messages being published directly on the dispatcher with some sort of new Dispatch function that takes in a proto.Envelope as an argument.
3. Hook up the two HandleIncomingWakuRelayMessage functions to publish directly to the dispatcher without Nats

Describe the uses cases for the feature

No response

Additional details

As a bonus, this change will remove one round of serialization for every message received from Waku.