Add filter_by_prefix to query_parameters schema
Add filter_by_prefix to query_parameters schema

This query parameter was introduced into Backdrop in January 2015

https://github.com/alphagov/backdrop/commit/6f56fa848a59046eb00b0b4893bf8f23e688b08f

Stagecraft has never been updated to allow modules to make use of the
parameter when querying for data. It is very useful for pulling out a
group of keys to then be run through the rate transform.

alphagov/stagecraft,alphagov/stagecraft,alphagov/stagecraft,alphagov/stagecraft