Support querying by parent or child of a data product

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:

```sql
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.

Begin by reviewing issue #26 and the existing query behavior for data products and metadata filters. Investigate how to retrieve children from filtered parents and parents from filtered children without assuming that general JOIN support is required. Done means both relationship directions support metadata-based filtering, with the chosen approach verified against the SQL examples.

Written by the indexing model from the issue text.