Support JSON encoding for Debezium topics

[chaas]

If you follow the Debezium Tutorial, the topics are populated with messages encoded using JSON. However, if you try to create a source using FORMAT JSON, you'll get the following error message:

materialize=> CREATE MATERIALIZED SOURCE IF NOT EXISTS customers FROM KAFKA BROKER 'kafka:9092' TOPIC 'debezium.inventory.customers' FORMAT JSON ENVELOPE DEBEZIUM;
ERROR:  JSON sources not yet supported

This means that users need to make sure that they set the following environment variables when configuring connect (with the appropriate schema registry URL, of course):

KEY_CONVERTER: io.confluent.connect.avro.AvroConverter
VALUE_CONVERTER: io.confluent.connect.avro.AvroConverter
CONNECT_KEY_CONVERTER_SCHEMA_REGISTRY_URL: http://schema-registry:8081
CONNECT_VALUE_CONVERTER_SCHEMA_REGISTRY_URL: http://schema-registry:8081

Supporting messages encoded using JSON would enable Materialize to work with "out of the box" Debezium configurations.

Note: this is not presently a priority. Production use cases are strongly encouraged to use Avro encoding instead, because:

The JSON converter includes the key and value schemas in every message, so it does produce very verbose events. Alternatively, you can use Apache Avro as a serialization format, which results in far smaller event messages. This is because it transforms each Kafka Connect schema into an Avro schema and stores the Avro schemas in a separate Schema Registry service. Thus, when the Avro converter serializes an event message, it places only a unique identifier for the schema along with an Avro-encoded binary representation of the value. As a result, the serialized messages that are transferred over the wire and stored in Kafka are far smaller than what you have seen here. In fact, the Avro Converter is able to use Avro schema evolution techniques to maintain the history of each schema in the Schema Registry.

—https://debezium.io/documentation/reference/tutorial.html