Agent plugin for Kafka 2.1.0+

[hsoftxl]

kafka client 2.3.1

[kezhenxu94]

https://github.com/apache/skywalking/blob/master/docs/en/setup/service-agent/java-agent/Supported-list.md

we don't support Kafka 2.3.x for now, if you want, please enhance the plugin and contribute, thanks

[kezhenxu94]

Confirmed, 2.3.1 is also supported, need to update the plugin list

[wu-sheng]

@hsoftxl Could you submit a pull request to update the doc?