Kafka Cloud events implementation does not provide support For Null, Ignore Key types

[Abuntxa]

The current extensión methods that allow transforming to and from CloudEvents into Kafka messages expects always messages of type Message<string?, byte[]> where the first generic template parameter is the type of Key in the Kafka messages.

When using the KafkaConsumer or producer it is usual to use the types "Confluent.Kafka.Ignore" or "Confluent.Kafka.Null" too that would handle messages like the following:
Message<Confluent.Kafka.Ignore, byte[]>
Message<Confluent.Kafka.Null, byte[]>

[jskeet]

This is something I would be unable to take on myself, not being familiar with Kafka. (I basically inherited the current code to maintain.) I'd welcome a pull request that addresses the concern, preferably without any breaking changes to the existing API.

[Abuntxa]

I have provided the following PR as an implementation with no breaking changes.