Add the ability to search a country by `ISO 3166-1 alpha-3` codes (e.g. USA, JPN, CHE)

### Issue Details

[You've added](https://github.com/AdguardTeam/AdGuardVPNForiOS/issues/234) `ISO 3166-1 alpha-2` country searches, but I would like to be able to use 3-letter abbreviations as well.

### Proposed solution

Add a country search using `ISO 3166-1 alpha-3` codes (e.g. USA, JPN, CHE).

Also, please make the country search for `ISO 3166-1` codes case-insensitive (e.g. so I can find the United States with `Us` and `us` queries).

### Alternative solution

_No response_