Add optional field with Debezium version to "source" element of messages

This will allow consumers to recognize the Debezium version used for creating a given message, helping them to adjust their behavior, e.g. in case of temporary workarounds and/or bugfixes.

Taking DBZ-578 as example, a consumer may currently adjust timestamps of snapshot messages to UTC (which they can if they happen to know the DB's timezone). It would have to stop that workaround once DBZ-578 is fixed. Today this would require to process all messages emitted by the old connector version before updating the consumer in a coordinated way. Having the knowledge about the emitting Debezium version as part of the messages themselves will allow to relax rollout of such fixes.