Add sell.account.rate_table.index endpoint

[yevgenko]

Add sell.account.rate_table.index operation to retrieve a seller's shipping rate tables.

Endpoint: GET /sell/account/v1/rate_table?marketplace_id={marketplace_id}

Docs: https://developer.ebay.com/api-docs/sell/account/resources/rate_table/methods/getRateTables

Response example

{
 "rateTables": [
   {
     "rateTableId": "5048772018",
     "countryCode": "GB",
     "name": "Domestic_Default",
     "locality": "DOMESTIC"
   }
 ]
}

Usage

client.sell.account.rate_table.index(site: EbayAPI::Site["EBAY_GB"])