feat: sync handlers

[danielknell]

Description

Allow registering regular functions as handlers.

Context

Sometimes asynchronous options are not available for handling a request, to handle these cases it would be useful to be able to register regular functions as handlers instead of co-routines.

@registry.subscribe_to(CreateWidget)
def handler(command: CreateWidget, bus: banshee.SyncBus) -> None:
    # synchronous task here
    ...

    bus.handle(WidgetCreated(id=command.id))

Possible implementation

Update the Registry class to allow synchronous functions to be subscribed to requests, and wrap them in a thread pool before registering via loop.run_in_executor.

These synchronous handlers would also need a synchronous wrapper around the MessageBus class to then further publish events.

This will need additional work and documentation around the injector integration to minimise problems with sharing resources between threads.