Returns the canonical set of values the country field of ETF geographic
allocation (/v2/etfs/countries) can take.

Every country value served by the API appears in this list. The list is
complete and authoritative. Each entry is tagged by type:

• country: ISO 3166 country or territory, including common aliases
• region: multi-country grouping such as "Europe" or "Emerging Markets"
• supranational: non-sovereign issuer bucket such as "Supranational" or
  "European Union", common in bond ETFs (World Bank, EIB, EU)
• other: non-geographic values such as "Cash", "Other", "Unknown", "N/A"

No ETF identifier is required.