Now that SortPattern is exposed, and we have a type-safe representation of a sort pattern, we should use it in the query planner.

There are many places where we use raw BSON in the planner.

This work would involve creating a SortPattern as part of canonicalization, and hanging it off of the CanonicalQuery. Then, updating all callers of QueryRequest::getSort() in the query planner to use the sort pattern instead.