Add RawProduce API

[apacker]

This PR adds a new RawProduce API to the client. The background/context for this API is: we have an internal system that does buffering of Kafka messages on disk, and needs to flush that data to Kafka without having to decode/re-encode the data and incur a 2x cost of compression/encoding. In this context, it would be optimal to be able to pass a raw/pre-encoded RecordSet to the Produce API, hence the introduction of a new RawProduce interface.

Notes:

• The existing ProduceResponse type is reused as there's no reason to introduce a new response type.
• Since request/response types and their resulting encodings are linked directly to an API key, I've introduced a mechanism to be able to override types via a OverrideTypeMessage interface.

[petedannemann]

🚀 great work, two small comments for now. I'll think this over a bit more and re-review later this week