feat: Add the ability to decode the Kafka Avro key and not just the value

[steeleworker]

Is your feature request related to a problem? Please describe.

When decoding Avro Kafka messages using the schema registry, currently only the value is decoded. Although you can INCLUDE key it means the column is a bytea type. However the key in our data is avro encoded. This prevents us from building an index on any property that is part of the key and forces us pull everything back on the client side and decode.

Describe the solution you'd like

It would be great if we had the option to decode the kafka key which is in Avro format.

Describe alternatives you've considered

No response

Additional context

No response

[steeleworker]

@xiangjinwu Looks like you have contributed to some of the related code. What do you think?