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/hidemyass/index.go

@@ -0,0 +1,54 @@
+package hidemyass
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"regexp"
+	"strings"
+)
+
+var indexOpenvpnLinksRegex = regexp.MustCompile(`<a[ ]+href=".+\.ovpn">.+\.ovpn</a>`)
+
+func fetchIndex(ctx context.Context, client *http.Client, indexURL string) (
+	openvpnURLs []string, err error) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, indexURL, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	htmlCode, err := io.ReadAll(response.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	if !strings.HasSuffix(indexURL, "/") {
+		indexURL += "/"
+	}
+
+	lines := strings.Split(string(htmlCode), "\n")
+	for _, line := range lines {
+		found := indexOpenvpnLinksRegex.FindString(line)
+		if len(found) == 0 {
+			continue
+		}
+		const prefix = `.ovpn">`
+		const suffix = `</a>`
+		startIndex := strings.Index(found, prefix) + len(prefix)
+		endIndex := strings.Index(found, suffix)
+		filename := found[startIndex:endIndex]
+		openvpnURL := indexURL + filename
+		if !strings.HasSuffix(openvpnURL, ".ovpn") {
+			continue
+		}
+		openvpnURLs = append(openvpnURLs, openvpnURL)
+	}
+
+	return openvpnURLs, nil
+}