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/nordvpn/ip.go

@@ -0,0 +1,16 @@
+package nordvpn
+
+import (
+	"fmt"
+	"net"
+)
+
+func parseIPv4(s string) (ipv4 net.IP, err error) {
+	ip := net.ParseIP(s)
+	if ip == nil {
+		return nil, fmt.Errorf("%w: %q", ErrParseIP, s)
+	} else if ip.To4() == nil {
+		return nil, fmt.Errorf("%w: %s", ErrNotIPv4, ip)
+	}
+	return ip, nil
+}