[define] usage of CloudEvents' `dataschema` conflicts with the Confluent wire format

(i) This issue has been manually transferred from a former internal repository, as a private repository issue cannot be transferred to a public repository.

Context

CloudEvents' dataschema should contain a reference to the schema that has been used to write the cloud event. If the Confluent schema registry and it's SerDes is used, this information is encoded using the Confluent wire format directly in the key/value of the record.

Currently, it is not possible to write the schema ID into a custom record header when using the Confluent Schema Registry SerDes. An open issue exists: https://github.com/confluentinc/schema-registry/issues/881

Other SerDe Libraries support writing to a custom header: https://www.apicur.io/registry/docs/apicurio-registry/2.2.x/getting-started/assembly-configuring-kafka-client-serdes.html#registry-serdes-config-props_registry

Therefore we need to define how this should be handled.

Start with the CloudEvents dataschema specification and the linked Confluent wire-format documentation, then compare them with the referenced Confluent Schema Registry and Apicurio SerDes behavior. The work is done when the guidelines clearly define how dataschema should be represented when schema information is embedded in record keys or values rather than written to a custom header.