refactor(iroh): Modularize protocol (#2454)

## Description

Use a newly created macro crate
https://github.com/n0-computer/nested-enum-utils to allow for from/to
conversions for deeply nested enums.

This allows us to split the protocol enum into multiple parts without
changing anything about the basic concept of how things are handled.

## Breaking Changes

Weirdly, I think there should be none since the protocol is private.

## Notes & open questions

Obvious next steps would be
- [x] define the subsystem specific messages in submodules
- [x] publish the macro crate
- [x] (maybe) split up the handling of the different subsystems
- [x] (maybe) remove the subsystem prefixes from the messages

Not so obvious next steps (maybe next PR)
- Allow moving the subsystem protocols to different crates. This is
difficult because you can not set the target when generating the
conversions.

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.