feat: add BuildJsonApiQueryAsync for custom query execution (#79)

- Add `BuildJsonApiQueryAsync<T>()` method to `JsonApiController`
- Returns processed `IQueryable` without pagination for
exports/aggregations
- Add `JsonApiQueryResult<T>` result class with `Query`, `Parameters`,
`TotalCount`
- Add 17 tests covering filters, includes, sorting, count behavior
- Consolidate statistics docs into new `build-query.md`

Author: erlendellefsen