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/cyberghost/string.go

@@ -0,0 +1,15 @@
+package cyberghost
+
+import "github.com/qdm12/gluetun/internal/models"
+
+// Stringify converts servers to code string format.
+func Stringify(servers []models.CyberghostServer) (s string) {
+	s = "func CyberghostServers() []models.CyberghostServer {\n"
+	s += "	return []models.CyberghostServer{\n"
+	for _, server := range servers {
+		s += "		" + server.String() + ",\n"
+	}
+	s += "	}\n"
+	s += "}"
+	return s
+}