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/openvpn/fetch.go

@@ -0,0 +1,41 @@
+package openvpn
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+func FetchFile(ctx context.Context, client *http.Client, url string) (
+	host string, err error) {
+	b, err := fetchData(ctx, client, url)
+	if err != nil {
+		return "", err
+	}
+
+	const rejectIP = true
+	const rejectDomain = false
+	hosts := extractRemoteHosts(b, rejectIP, rejectDomain)
+	if len(hosts) == 0 {
+		return "", fmt.Errorf("%w for url %s", ErrNoRemoteHost, url)
+	}
+
+	return hosts[0], nil
+}
+
+func fetchData(ctx context.Context, client *http.Client, url string) (
+	b []byte, err error) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	return io.ReadAll(response.Body)
+}