Add extension methods for DependentProducerBuilder

[dstelljes]

Resolves #30.

[ClementeGao]

hi @dstelljes May I ask you a few questions？
What is the role of DependentProducerBuilder?
What is the difference between DependentProducerBuilder and ProducerBuilder?
When to use DependentProducerBuilder？

[dstelljes]

DependentProducerBuilder allows you to pass in an existing client handle: https://github.com/confluentinc/confluent-kafka-dotnet/blob/v2.3.0/src/Confluent.Kafka/DependentProducerBuilder.cs#L58-L66

This can be useful if you want to use the same underlying client with multiple .NET producers. For instance, say you have IProducer<string, Event1> and IProducer<string, Event2> producing to the same topic; sharing a handle enables the client to batch messages from both even though the types differ.