Allow setting of custom auth module for kafka sasl handler

[pthalasta]

Current versions of kafka output/input plugins do not provide a configuration to set sasl.client.callback.handler.class within the logstash config making it difficult to use kafka output/input with custom auth modules. Users can add custom jars to the lib folder of logstash or in the classpath and use this configuration to provide auth for their kafka env.

I've made changes to the files locally and tested on the steps that need to be done within the plugin's ruby file where the consumer/producer is created. Happy to contribute and open PR if required.