[WIP] feat: protocol handlers

[Frando]

Description

Notes & open questions

Change checklist

• [ ] Self-review.
• [ ] Documentation updates if relevant.
• [ ] Tests if relevant.

[Frando]

This does not work out well this way. Instead our current idea is to expose the endpoint events not as a single callback, but as an (overflowing, non-lagging) broadcast stream. This would very much simplify the composition of protocols, as users won't have to merge their event listeners into a single callback, which needs to be passed to MagicEndpointBuilder (thus before MagicEndpoint exists, which makes the current state lead to chicken-and-egg problems)

[dignifiedquire]

https://github.com/n0-computer/iroh/pull/1847 should help to improve the startup story

[Frando]

Closing this. Needs more thought and is not on the immediate agenda.