Remove StartAsync and StopAsync from IEventBus

[danielcirket]

We should decouple these from the IEventBus implementation, we shouldn't require you to implement a start and stop feature as they're not always going to apply.

For example, using Azure service bus, you can publish a message but don't necessarily have to be subscribed to a topic which causes you to receive messages as a consumer, this could also apply the other way around where you're purely a consumer and not producing any messages.

In the ServiceBus package, we already have this distinction with the sender/receivers, and use those within the concrete implementation of IEventBus, so perhaps this should be the case with the base abstractions too.

Should this actually become along the lines of:

public interface IEventBusPublisher
{
    Task PublishAsync<TEvent>(TEvent @event) where TEvent : IEvent;
    Task PublishAsync(IEnumerable<IEvent> events);
    Task<string> PublishAsync<TEvent>(TEvent @event, DateTimeOffset publishOnUtc) where TEvent : IEvent;
    // NOTE(Dan): What should happen to cancelling a message?
    Task CancelScheduledMessageAsync(string identifier);
}

public interface IEventBusReceiver
{
    Task StartAsync(/* Should we accept a cancellation token? */);
    Task StopAsync(/* Should we accept a cancellation token? */);
}