Very early stage non functional prototype to add support for any kind of
subscriptions, aiming for NUT-17 support

There are two functions, an async and a fire and forget version

I do not like `unwrap()`

This implementation does not rely on any new crate, instead it implements the
types for the WebSocket.

This is WIP, so this commit will be rebased and keep changing

Also dropped `tokio::select` since it is not stable in 1.63

Instead of letting Axum create it and use it for their WebSocket, make it part
of Mint so that event subscriptions and publishing can happen in all layers.

Also added helper functions to emit events

Some typos in comments were fixed in this commit as well

I think the `subscription_manager` was a bit unintuitive; I think
`pubsub_manager` is a much more descriptive name.