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/surfshark/api.go

@@ -0,0 +1,53 @@
+package surfshark
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+var (
+	ErrHTTPStatusCodeNotOK   = errors.New("HTTP status code not OK")
+	ErrUnmarshalResponseBody = errors.New("failed unmarshaling response body")
+)
+
+//nolint:unused
+type serverData struct {
+	Host     string `json:"connectionName"`
+	Country  string `json:"country"`
+	Location string `json:"location"`
+}
+
+//nolint:unused,deadcode
+func fetchAPI(ctx context.Context, client *http.Client) (
+	servers []serverData, err error) {
+	const url = "https://my.surfshark.com/vpn/api/v4/server/clusters"
+
+	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()
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%w: %s", ErrHTTPStatusCodeNotOK, response.Status)
+	}
+
+	decoder := json.NewDecoder(response.Body)
+	if err := decoder.Decode(&servers); err != nil {
+		return nil, fmt.Errorf("%w: %s", ErrUnmarshalResponseBody, err)
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return nil, err
+	}
+
+	return servers, nil
+}