Create an Extractor that uses a generator callback

[kdebisschop]

The idea here is to use a Generator callback to yield multiple rows for each row of an input.

In particular, can use $pipeline->extract(...)->toIterator() as input to this Extractor. In such a configuration, two extractors can be chained together to return N x M rows selecting N rows of JSON arrays from an SQL database, each containing M elements in the JSON array.

I have this code and a tests, working on docs now.