Automatically fill fields from input step at runtime when the fields are not specified

[juarezr]

• Uses cases:
  1. use the same transformation for exporting for diferent sources
  2. switching between output formats according the needs (eg: avro, csv, parquet)
• Reproducing the concept:
  1. Create a extract transformation with a final step of type 'Copy rows to result'
  2. Create a output transformation with a initial step of type 'Get rows from result'
  3. Connect the initial step to a second step of type 'Avro Output'
  4. Create a Job and connect the extract transformation to the output transformation
  5. Today the Avro Output plugin create a Avro file with zero columns.
  6. It should fill the columns at runtime inspecting the columns in the previous step
• Problems:
  • If the Fields are not specified in the fields tab, Avro Output plugin creates a file with no columns, even the the previous step has fields defined at runtime.
  • If the Avro Output step are replaced by a 'Text file output', the resulting file is created with all columns correct.