Create a code sample and an intergration test with protobuf and schema registry

[jerrinot]

We do not have any sample with protobuf. Protobuf is a popular format, we should add:

1. Integration test, similar to Avro we already have:

   kafka-questdb-connector/integration-tests/avro-schema-registry/src/test/java/io/questdb/kafka/AvroSchemaRegistryIT.java

   Line 36 in 452807e

   public class AvroSchemaRegistryIT {

2. Easy to follow step-by-step guide showing how end-users can configured the connector for Protobuf with a schema registry.