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/mullvad/ips.go

@@ -0,0 +1,30 @@
+package mullvad
+
+import (
+	"bytes"
+	"net"
+	"sort"
+)
+
+func uniqueSortedIPs(ips []net.IP) []net.IP {
+	uniqueIPs := make(map[string]struct{}, len(ips))
+	for _, ip := range ips {
+		key := ip.String()
+		uniqueIPs[key] = struct{}{}
+	}
+
+	ips = make([]net.IP, 0, len(uniqueIPs))
+	for key := range uniqueIPs {
+		ip := net.ParseIP(key)
+		if ipv4 := ip.To4(); ipv4 != nil {
+			ip = ipv4
+		}
+		ips = append(ips, ip)
+	}
+
+	sort.Slice(ips, func(i, j int) bool {
+		return bytes.Compare(ips[i], ips[j]) < 0
+	})
+
+	return ips
+}