Allow configuration of 'PrefetchCount' value while configuring AzureServiceBus messagepump

[fgheysels]

When we configure an Azure ServiceBus message-pump, it is possible to specify certain configuration options like MaxConcurrentCalls etc...
However, it doesn't seem to be possible to configure the PrefetchCount setting.

It would be nice if we could configure this.

https://learn.microsoft.com/en-us/dotnet/api/azure.messaging.servicebus.servicebusprocessoroptions.prefetchcount?view=azure-dotnet