Changes to Get countries list (base pricing)

the pagination/page_size response property's max was increased from 150.00 to 250.00 for the response status 200

for the query request parameter page_size, the max was increased from 150.00 to 250.00

added the non-success response with the status 400

added the non-success response with the status 500

added the required property items/items/base_price to the response with the 200 status

added the required property items/items/popularity_index to the response with the 200 status