Kafka Connect: Add the configuration option to provide a transactional id prefix to use

[thjaeckle]

In a Kafka setup where strict ACLs are applied, e.g. based on a certain PREFIX for Transactional IDs, it is important to have the prefix configurable.

Currently, it is the name of the "Channel" (e.g. "worker" or "coordinator" for the 2 extending subclasses):

iceberg/kafka-connect/kafka-connect/src/main/java/org/apache/iceberg/connect/channel/Channel.java

Lines 57 to 67 in 540d6a6

	Channel(
	String name,
	String consumerGroupId,
	IcebergSinkConfig config,
	KafkaClientFactory clientFactory,
	SinkTaskContext context) {
	this.controlTopic = config.controlTopic();
	this.connectGroupId = config.connectGroupId();
	this.context = context;
	String transactionalId = name + config.transactionalSuffix();

This PR introduces an optional prefix to add - which by default is empty so that the behaviour is not changed to the current behaviour.