[Schemas] Decode raw data in records with schemas before passing them to processors

[lovromazgon]

Records coming from newer connectors will contain raw data with avro encoded bytes, while the schema subject and version will be attached in the metadata. If that's the case, we can actually decode that raw data into structured data before giving the record to a processor. After the processor is done we can again encode the data and update the schema if needed (if it changed).