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/mullvad/api.go

@@ -0,0 +1,55 @@
+package mullvad
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+var (
+	ErrHTTPStatusCodeNotOK   = errors.New("HTTP status code not OK")
+	ErrUnmarshalResponseBody = errors.New("failed unmarshaling response body")
+)
+
+type serverData struct {
+	Hostname string `json:"hostname"`
+	Country  string `json:"country_name"`
+	City     string `json:"city_name"`
+	Active   bool   `json:"active"`
+	Owned    bool   `json:"owned"`
+	Provider string `json:"provider"`
+	IPv4     string `json:"ipv4_addr_in"`
+	IPv6     string `json:"ipv6_addr_in"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (data []serverData, err error) {
+	const url = "https://api.mullvad.net/www/relays/openvpn/"
+
+	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(&data); err != nil {
+		return nil, fmt.Errorf("%w: %s", ErrUnmarshalResponseBody, err)
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return nil, err
+	}
+
+	return data, nil
+}