Add support for flexible messages

[yolken-segment]

Description

This change updates kafka-go to support the "flexible" message format described in KIP-482. Among other changes, the new format adds optional "tagged fields" into API messages and also makes the encodings of strings and arrays more compact.

The values of the tagged fields aren't used for much yet (I think), but we need to support the new, "flexible" format to get the latest API versions described in the Kafka protocol guide.

The proposed interface is to support tagged fields via a new, tag= property in the existing protocol struct tags, e.g.:

type Request struct {
    [required fields]

    TaggedField0 `kafka:"min=v1,max=v3,tag=0"`
    TaggedField1 `kafka:"min=v2,max=v3,tag=1"`
   ...
}

As an example and to provide something to test with, this change updates the max version of the CreateTopics API to v5, which uses the new format.

Misc. notes and TODOs

1. I've added Kafka 2.4.1 into the CI since something >= 2.4 is needed to exercise the new message formats. However, the nettest suite fails for this version, so I'm temporarily adding an option to skip this. I get the same failures when running from the base 0.4 branch, so I don't think these errors are related to the changes here.
2. KIP-482 defines new, "compact" encodings that are different from the ones currently defined in the kafka-go procotol. I've renamed the latter from "compact" to "var", e.g. VarString, VarBytes, etc. to distinguish them.
3. Non-empty tag buffers aren't included yet in any real APIs (as far as I can tell), so it's hard to be sure that the implementation here is 100% aligned with what Kafka will expect in the future. We may need to make fixes in the future once tagged fields are actively set.

[achille-roussel]

Do you mind explaining why these tests cannot pass with 2.4?

[yolken-segment]

Done. Also added a similar comment to where KAFKA_SKIP_NETTEST is read in the code.

[achille-roussel]

This is looking great 🙌