Move to Wasi Messaging types

[lee-orr]

Adapt types to be based on wasi messaging - https://github.com/WebAssembly/wasi-messaging/blob/main/wit/handler.wit

[arnonuem]

This link points to 404. There is no handler.wit anymore - looks like it was refactored to types.wit

[lee-orr]

Thanks for pointing that out - the updated spec is here: https://github.com/WebAssembly/wasi-messaging/tree/main

This is less useful ATM for spin, because it involves the guest hard coding their configuration.... but some other elements might still be useful.