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/name.go

@@ -0,0 +1,29 @@
+package nordvpn
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+var (
+	ErrNoIDInServerName      = errors.New("no ID in server name")
+	ErrInvalidIDInServerName = errors.New("invalid ID in server name")
+)
+
+func parseServerName(serverName string) (number uint16, err error) {
+	i := strings.IndexRune(serverName, '#')
+	if i < 0 {
+		return 0, fmt.Errorf("%w: %s", ErrNoIDInServerName, serverName)
+	}
+
+	idString := serverName[i+1:]
+	idUint64, err := strconv.ParseUint(idString, 10, 16)
+	if err != nil {
+		return 0, fmt.Errorf("%w: %s", ErrInvalidIDInServerName, serverName)
+	}
+
+	number = uint16(idUint64)
+	return number, nil
+}