Always cast on field extraction

This is a courtesy to the reader, who has no idea what to expect when reading your transform.

Bad OPAL:

make_col
  id:FIELDS.container.id,
  name:FIELDS.container.name,
  labels:FIELDS.container.labels,
  imageName:FIELDS.container.image.name

Good OPAL:

make_col
  id:string(FIELDS.container.id),
  name:string(FIELDS.container.name),
  labels:object(FIELDS.container.labels),
  imageName:string(FIELDS.container.image.name)