feat(products): eager load values in product item filters query

## Context

A product item can have thousands of filters, each serialized with its
values and their billable metric filters. Resolving those per row causes an
N+1 on every paginated page.

## Description

Eager load values and their billable_metric_filter in
ProductItemFiltersQuery so both the GraphQL and REST list endpoints avoid
the N+1 when rendering filter values.

Author: rsempe

app/queries/product_item_filters_query.rb

@@ -5,7 +5,7 @@ class ProductItemFiltersQuery < BaseQuery
   Filters = BaseFilters[:product_item_id]
 
   def call
-    product_item_filters = base_scope.result
+    product_item_filters = base_scope.result.includes(values: :billable_metric_filter)
     product_item_filters = paginate(product_item_filters)
     product_item_filters = apply_consistent_ordering(product_item_filters)