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/windscribe/api.go

@@ -0,0 +1,61 @@
+package windscribe
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"strconv"
+	"time"
+)
+
+var (
+	ErrHTTPStatusCodeNotOK   = errors.New("HTTP status code not OK")
+	ErrUnmarshalResponseBody = errors.New("failed unmarshaling response body")
+)
+
+type apiData struct {
+	Data []regionData `json:"data"`
+}
+
+type regionData struct {
+	Region string      `json:"name"`
+	Groups []groupData `json:"groups"`
+}
+
+type groupData struct {
+	City  string       `json:"city"`
+	Nodes []serverData `json:"nodes"`
+}
+
+type serverData struct {
+	Hostname  string `json:"hostname"`
+	OpenvpnIP net.IP `json:"ip2"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (
+	data apiData, err error) {
+	const baseURL = "https://assets.windscribe.com/serverlist/mob-v2/1/"
+	cacheBreaker := time.Now().Unix()
+	url := baseURL + strconv.Itoa(int(cacheBreaker))
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return data, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return data, err
+	}
+	defer response.Body.Close()
+
+	decoder := json.NewDecoder(response.Body)
+	if err := decoder.Decode(&data); err != nil {
+		return data, fmt.Errorf("%w: %s", ErrUnmarshalResponseBody, err)
+	}
+
+	return data, nil
+}