Builder: add substream chunks/slices

[dgram]

As i've seen currently its only possible to use substreams once by once.

Example:
Parent Stream, reads/returns multiple products
Sub-Stream, can currently only handle one product request id once-by-once instead of get for example 100 product ids and handle them as one request.

https://api.example.com/product_details?product_ids={{ stream_slice.100_event_ids }}