code: public static Shape getPayloadShape(Map<String, Shape> c2jShapes, Shape outputShape) {
          if (outputShape.getPayload() == null) {
              return null;
          }
          Member payloadMember = outputShape.getMembers().get(outputShape.getPayload());
          return c2jShapes.get(payloadMember.getShape());
      }
docs: If there is a member in the output shape that is explicitly marked as the payload (with the
  payload trait) this method returns the target shape of that member. Otherwise this method
  returns null.
  
  @param c2jShapes
  All C2J shapes
  @param outputShape
  Output shape of operation that may contain a member designated as the payload