support new compound shard key handling of Debezium Source Connector

When processing CDC events coming from Debezium's Source Connector for MongoDB there was a recent update which affects the handling how to encode the shard key of sharded collections in the payload. The corresponding PR was merged by now https://issues.redhat.com/browse/DBZ-1781.
Implement support for this in the sink connector according to the documentation which can be found in the DBZ docs.