[Kafka Test Companion] Inconsistent handling of properties

[Abaudat]

The problem

In Kafka Test Companion's ProducerBuilder and ConsumerBuilder, the properties seem to be treated inconsistently:

• The constructors of both ProducerBuilder and ConsumerBuilder use the signature Map<String, Object>
• The properties field in both ProducerBuilder and ConsumerBuilder also use the signature Map<String, Object>
• The method ConsumerBuilder#withProps also use the signature Map<String, Object>

However:

• The method ProducerBuilder#withProps uses the signature Map<String, String>
• Both ProducerBuilder#withProp and ConsumerBuilder#withProp use the type String for the value

As both KafkaConsumer and KafkaProducer support properties of the form Map<String, Object>, it seems like ProducerBuilder and ConsumerBuilder should use this form instead of restricting the property value to String.

Suggestion of implementation

• Change ProducerBuilder#withProps to take a Map<String, Object>
• Change both ProducerBuilder#withProp and ConsumerBuilder#withProp to use the type Object for the value

I am willing to work on the PR, I just need confirmation that this change would be welcome.