Add support for flex versions

[dearchap]

Flex version support as per the kafka protocol specification and as implemented in the confluent librdkafka library

https://github.com/edenhill/librdkafka/blob/master/src/rdkafka_buf.h

[dearchap]

@travisjeffery I just wanted to show the initial design before I add unit tests for some common messages(apiVersions, produce and so on)

[twmb]

It might be worth it to check out my generated protocol definitions that implement serializing and deserializing all requests/responses through all versions: https://github.com/twmb/franz-go/blob/master/pkg/kmsg/generated.go