Add test cases for different handshake versions

[kjellwinblad]

There are three handshake versions undefined, 0 and 1 (see #5 (comment)). @starbelly, please correct me if I did not understand this correctly. We should add Kafka test cases and github actions for all handshake versions by extending the code in the example directory.

[starbelly]

@kjellwinblad That's correct. With auth/8 function added to kafka_protocol that will invoke auth/7 on a auth plugin, we'll now get handshake 0 or 1. We may also get undefined from ourselves if auth/6 is invoked from kafka_protocol and we set it ourselves (to undefined).

However, we currently don't support handshake v0. See issue #8.