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/fastestvpn/filename.go

@@ -0,0 +1,39 @@
+package fastestvpn
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+var errFilenameNoProtocolSuffix = errors.New("filename does not have a protocol suffix")
+
+var trailNumberExp = regexp.MustCompile(`[0-9]+$`)
+
+func parseFilename(fileName string) (
+	country string, tcp, udp bool, err error,
+) {
+	const (
+		tcpSuffix = "-TCP.ovpn"
+		udpSuffix = "-UDP.ovpn"
+	)
+	var suffix string
+	switch {
+	case strings.HasSuffix(fileName, tcpSuffix):
+		suffix = tcpSuffix
+		tcp = true
+	case strings.HasSuffix(fileName, udpSuffix):
+		suffix = udpSuffix
+		udp = true
+	default:
+		return "", false, false, fmt.Errorf("%w: %s",
+			errFilenameNoProtocolSuffix, fileName)
+	}
+
+	countryWithNumber := strings.TrimSuffix(fileName, suffix)
+	number := trailNumberExp.FindString(countryWithNumber)
+	country = countryWithNumber[:len(countryWithNumber)-len(number)]
+
+	return country, tcp, udp, nil
+}