AC:
- Description field is added under the item value type
- Descriptions can be asked for in any language code available in Wikidata
Task breakdown notes:
- change the ItemValue schema in repo/WikibaseRepo.datatypes.php to include the description field
- use ItemDescriptionsResolver in the resolver of the new description field, similar to how the field is defined in Types::getItemSearchResultNodeType()
- add integration tests to ItemQueryTest