feat: Abstract confluent builders into factories

[robertcoltheart]

Description

Fixes #623

Moves the Confluent producer/consumer builder logic into abstractions that can provide an integration point for an in-memory broker, for use in unit and integration testing.

Currently we need to run a real Kafka server (or docker container) to run our testing suite, but it would be nicer (and faster) if we could control the broker configuration in such a way to move messages around in memory without needing a live Kafka server.

A follow up change to this could be an in-memory broker Nuget package that can be used for testing purposes.

The use case for this would be something like this:

var services = new ServiceCollection()
    .AddKafka(...);

services.AddSingleton<IProducerBuilderFactory, InMemoryProducerBuilderFactory>(); // Replaces the default factory

public class InMemoryProducerBuilderFactory : IProducerBuilderFactory
{
    public IProducerBuilder CreateProducerBuilder(ProducerConfig config)
    {
        return new InMemoryProducerBuilder();
    }
}

public class InMemoryProducerBuilder : IProducerBuilder
{
    public IProducer<byte[], byte[]> Build()
    {
        return new InMemoryProducer();
    }
}

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have made corresponding changes to the documentation

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