feat(irpc-iroh): add iroh ProtocolHandler, restructure example

[Frando]

• Adds a IrohProtocol<R> to irpc_iroh that implements iroh::router::ProtocolHandler, which makes it easy to use this with an iroh Router
• Restructures the iroh example to use the protocol. Also moves the service implementation into a module to demonstrate that you can keep the implementation private easily.

[rklaehn]

This is nice. So we can use irpc as a nice way to do iroh protocols.