Let ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG be overridable by user in KafkaSourceBuilder

This issue is a follow-up to this mailing list discussion. 

I'd like to propose letting the ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG be overridable by user in KafkaSourceBuilder, as shown in this DRAFT PR:
 
https://github.com/apache/flink-connector-kafka/pull/108
 
From the PR description: 

This allows users to easily implement the claim check large message pattern without bringing any concerns into the Flink codebase otherwise, by specifying a value.deserializer that handles it, but otherwise passes through the bytes.
Note: overriding value.serializer is already supported on the Producer side: 

https://github.com/apache/flink-connector-kafka/blob/15d3fbd4e65dae6c334e2386dd337d2bf423c216/flink-connector-kafka/src/main/java/org/apache/flink/connector/kafka/sink/KafkaSinkBuilder.java#L82-L83

 
Other Reading:
https://www.kai-waehner.de/blog/2020/08/07/apache-kafka-handling-large-messages-and-files-for-image-video-audio-processing/

https://www.conduktor.io/kafka/how-to-send-large-messages-in-apache-kafka/#Option-1:-using-an-external-store-(GB-size-messages)-0

 
What do folks think? If it seems reasonable I can follow the steps in the contribution guide.