Add support for EventHubs PluggableCheckpointStoreEventProcessor

Azure EventHubs has support for a base class for creating an event processor with custom processing logic to consume events. See https://learn.microsoft.com/en-us/dotnet/api/azure.messaging.eventhubs.primitives.pluggablecheckpointstoreeventprocessor-1?view=azure-dotnet.

We don't have support for this in our Azure EventHubs client integration.

A design consideration is that since this is a base class, developers will need to derive their own class. We will need to figure out which constructors are available on the derived class, and pick the most appropriate one.

One other consideration here is that users of PluggableCheckpointStoreEventProcessor can create multiple instances and coordinate sharing partitions between them. As such, adding a single instance into DI doesn't support this scenario. So we may need to consider how to support it (if at all). It's possible the API doesn't add the processor to DI, but instead just return one when called.