Maintenance: refactor servers updater code

- Require at least 80% of number of servers now to pass
- Each provider is in its own package with a common structure
- Unzip package with unzipper interface
- Openvpn package with extraction and download functions

internal/updater/providers/protonvpn/countries.go

@@ -0,0 +1,14 @@
+package protonvpn
+
+import "strings"
+
+func codeToCountry(countryCode string, countryCodes map[string]string) (
+	country string, warning string) {
+	countryCode = strings.ToLower(countryCode)
+	country, ok := countryCodes[countryCode]
+	if !ok {
+		warning = "unknown country code: " + countryCode
+		country = countryCode
+	}
+	return country, warning
+}