Provide outbox routing example for MongoDB

DBZ-3528 provided a new SMT for routing outbox events from a MongoDB instance. Similar to the existing example for the relational routing SMT, there should be a full example showing the usage. There are several options:

• Add another source service to the existing example which uses MongoDB
• Duplicate the entire existing example, porting the source service from RDBMS to MongoDB
• Have a basic new example without actual source and sink services, but which simply shows the insertion into the outbox collection in a transaction using the MongoDB CLI and consumption of the events using kafka-console-consumer.sh