custom resource finder

[fkowal]

Context:
When doing CSV export
It's quite common to have business requirements, to add some aggregate/external fields.

ie

shops + orders_aggregate { count }

I tried several approaches and one i'd like to enable

1. <ExportButton resource="shops_with_orders_report" /> + buildField

where shops_with_orders_report is an "alias" that only exists in the application
and the resource that will be used is the shops
and a custom buildField will be provided and will decorate the shops data with extra gql { orders_aggregate { count }}