Get company officers (cached)

Every appointment at a company, current and resigned, read from Companies House (every page) and cached; served from the cache while it is under 24 hours old. A person re-appointed to the same role has a row for each appointment. When Companies House cannot be reached, the cached copy is served however old (_stale: true, with _cached_at); with no copy held the response is 503 with code companies_house_unavailable and Retry-After.