Support querying by parent or child of a data product

[machristie]

The use case is that you want to get all of the children of parents that have certain metadata values. Or you want to get all of the parents of children that have certain metadata values.

For example, getting children when filtering by parents:

select child.*
from my_schema child
inner join my_other_schema parent
parent.data_product_id = child.parent_data_product_id
where parent.field1 = 'abc'
-- etc.

See also #26 which deals with general JOIN support. Note that for this issue, supporting a JOIN is not strictly necessary. Some investigation is needed to find the best way to satisfy the requirement.