Clarify how to add a custom `value_serializerer`

[yeikel]

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The current documentation is very vague about the value_serializerer field.

While it says that the default is org.apache.kafka.common.serialization.StringSerializer it does not explain in any way how to customize this value beyond the default Kafka types (ie: com.mycustom.serialization.Serializer)

URL:

Example: https://www.elastic.co/guide/en/logstash/current/plugins-outputs-kafka.html#plugins-outputs-kafka-value_serializer