Add support for sourcePartition and sourceOffset

[oscerd]

https://github.com/jboss-fuse/camel-kafka-connector/blob/master/core/src/main/java/org/apache/camel/kafkaconnector/CamelSourceTask.java#L100

[valdar]

Source camel Kafka connectors do not set sourcePartition and sourceOffset.

A way to easily do that in each connector should be implemented in the core module. Each connector can then implement his own specific way.

Another approach that can be explored is to provide such a generic mechanism in camel itself since resuming execution from a "saved" point is a feature of broader interest than just in the Kafka context. CC: @lburgazzoli