[Bug] Cannot use two different types of ProducerRegistry

[iancooper]

Describe the bug

If you want to use two different types of ProducerRegistry, this is not possible. We create the ProducerRegistry from the Producers that you want to register with us, via a Factory. Because the Factory is tied to the Transport (Kafka, RMQ) etc. you can't have a ProducerRegistry that contains multiple transports

Suggested Fix

We should enable the merging of Transport specific Producer Registries into a single Producer Registry. To do this we should add a Merge extension method for a Producer Registry, that takes another Producer Registry and returns a merged Producer Registry. By using an extension method we should be able to add this to both V9 and V10