raw and custom scan

[rapimo]

this adds two new scan methods

• raw simply dumps the message into a single column with only using the columns type in function
• custom allows the user to define a custom decoding function / dynamic lib which will be automatically loaded, the function kafka_custom_msglen is added here as a rather useless example

this also add the experimental expandKafkaScanPData methods which would split the expandKafkaScanPData into chunks of batch size allowing multiple parallel workers to do the job better, but this seems not to be very fruitfull