Public country list endpoints

Added two unauthenticated v2 endpoints that return the countries CoinGate currently supports per user category. Useful for building country pickers, restricting signup forms, and prechecking eligibility before sending users into a flow.

• GET /v2/countries/shoppers — countries where users can pay for orders.
• GET /v2/countries/merchants — countries where users can sign up and onboard as merchants.

Both endpoints return a JSON array of { alpha2_code, alpha3_code, name } objects, sorted alphabetically by name. Restricted countries (sanctions, etc.) are excluded automatically. Names are returned in English.

curl https://api.coingate.com/v2/countries/shoppers

[
  { "alpha2_code": "DE", "alpha3_code": "DEU", "name": "Germany" },
  { "alpha2_code": "LT", "alpha3_code": "LTU", "name": "Lithuania" },
  { "alpha2_code": "US", "alpha3_code": "USA", "name": "United States" }
]