#534: add featured collections (by volume) query

[austinmilt]

This PR adds a query to get collections ordered by volume (sum of purchases in collection).

Changes

• added enums to allow setting the order direction on queries as a Graph QL variable (custom_types.rs, query_root.rs)
• added SQL query to get collections ordered by volume over a time window (collections.rs)
• added root Graph QL query for collections by volume over a time window (query_root.rs)

related #534

[austinmilt]

Looks good. Can we propagate the new pre_query_search_limit parameter to the other Meilisearch queries in query_root.rs?

@ray-kast The pre_query_search_limit should only be used when passing the results of Meilisearch to an SQL query. That's because this limit is a buffer to help increase the odds of getting consistent results on the SQL query after leveraging Meilisearch. Otherwise, the limit and offset provided in the API should be passed direct to Meilisearch. Currently the other search endpoints have limits and offsets, so no change needs to be made there.