Add support for column projection to parquet sources

[isnotinvain]

This is similar to #1050, but adds another method .withColumns(...) to parquet sources for specifying projection push down.

[johnynek]

extends AnyVal should work here.

[johnynek]

This seems fine to me, but I think we should include an example, even in the comments, of making a source where inside the job the user can pass a filter (so, companion object or constructor takes an optional filter argument).

[isnotinvain]

The reason I made this a trait not a constructor arg was so that you can use this feature even if the Source you want to add a filter predicate to doesn't have a constructor arg for the filter predicate.

For example:

// note, MyCustomSource does not take a filter as an argument
class MyCustomSource(dateRange: DateRange) 
  extends DailySuffixParquetThrift[SomeTBase]("/foo/bar", dateRange)

val customSourceWithFilter = new MyCustomSource(dr) { 
  override val filterPredicate = Some(fp) 
}

I can add an example of this. Do you think I should add constructor params as well? That can be done like this:

class MyCustomSource(
  dateRange: DateRange, 
  override val filterPredicate: Option[FilterPredicate] = None
) extends DailySuffixParquetThrift[SomeTBase]("/foo/bar", dateRange)

[johnynek]

this doc describes setting a key in the config. Is this how this works under the covers? What about multi-input (merges, joins)?

[isnotinvain]

this is setup in sourceConfInit which handles this case