As a gadget developer I want to read data of a single statement so that I can use it my gadget.
return a json representation of the statement as returned by the /entities/items/<item_id> endpoint for a single statement in the statements section.
error handling:
- 400 - invalid statement ID including statement IDs of statements not on Items
- 404 - statement not found
- 500 - unexpected
Spec: https://wmde.github.io/wikibase-rest-api-proposal/#/statements/get_statements__statement_id_