code: public Optional<PartnerSubscription> show(long partnerId, long accountId, long subscriptionId)
      {
          return HTTP.GET(String.format("/v2/partners/%d/accounts/%d/subscriptions/%d", partnerId, accountId, subscriptionId), PARTNER_SUBSCRIPTION);
      }
docs: Returns the subscription with the given id.
  @param partnerId The id of the partner the subscription belongs to
  @param accountId The id of the account for the subscription
  @param subscriptionId The id of the subscription to return
  @return The subscription