Use Sangria Deferred for APIs that support filtering

[nightscape]

Afais there is no way to evade the n+1 requests problem with the current API.
I was wondering if it would be possible to either enhance the existing http directive or provide a new directive that uses Sangria's deferred resolvers mechanism to query APIs (in our case OData) that allow filtering and could therefore return all required (nested) elements at once.
Do you think this is possible?
We would be happy to provide a PR if you can provide some pointers how to start 😃

[nightscape]

Started hacking away in #6