Include encrypted data key in Kafka buffer message.

[dlvenable]

Coming from #3620, include metadata about encryption in the Kafka buffer message

message BufferedData {
   /* The format of the message as it was written.
    */
   MessageFormat message_format = 1;
  
  /* The actual data. This is encrypted if key_id is present. Otherwise, it
   * is unencrypted data.
   */
  bytes data = 2;

  /* Indicates if data is encrypted or not.
   */
  optional boolean encrypted = 3;
   
   /* The data key which encrypted the data field. This will be encrypted.
    * The consuming Data Prepper node must have the ability to decrypt this key.
    */
  optional bytes encrypted_data_key = 4;
}