This is the first GraphQL query from #151

 that provides some more
significant querying of data, and so requires we wire in a new sqlc
querier.

This introduces queries that are largely taken from other places, to
retrieve the core data required by the query.

We can make sure to create lighter-weight queries for `Count`ing data,
as well as provide a helper method to convert DB types to GraphQL types.

To make sure that we can reference values that are having pointers
pointed to them, we need to make sure we handle loopvars correctly.

Co-authored-by: Keith Lyall <keith.lyall@deliveroo.co.uk>