MINOR: Adding client Base64 serializer and deserializer

[LatentLag]

Adding client serialization for Base64 which enables two use cases. First, these allow messages encoded in base64 to not incur the encoding penalty of a larger file size. Second, binary encoded messages may be used more simply on the command line.

This is a minor change. For testing the class files were added to an existing kafka-client.jar and exercised with kafka-console-producer.sh and kafka-console-consumer.sh in Windows and Ubuntu 20.04.

This is my own work and I license it to this project.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)

[dajac]

@LatentLag Thanks for the PR. I think that we would need a KIP for adding those because I suppose that we consider them as public interfaces. The process is described here: https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals.