Add SASL and PLAIN support

[lukel97]

• Adds the base for a SASL authentication API
• Comes with the PLAIN mechanism
• Cluster can now create topics
• Rework the docker tests to be more reliable and extensible
• Refactor many internal types to be structs
• Add an associated type on the KafkaRequest protocol for the associated KafkaResponse
• Make Connection.write(request:_) generic to this associated type
• Tidy up a lot more internals