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/servers.go

@@ -0,0 +1,47 @@
+// Package windscribe contains code to obtain the server information
+// for the Windscribe provider.
+package windscribe
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+var ErrNotEnoughServers = errors.New("not enough servers found")
+
+func GetServers(ctx context.Context, client *http.Client, minServers int) (
+	servers []models.WindscribeServer, err error) {
+	data, err := fetchAPI(ctx, client)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, regionData := range data.Data {
+		region := regionData.Region
+		for _, group := range regionData.Groups {
+			city := group.City
+			for _, node := range group.Nodes {
+				server := models.WindscribeServer{
+					Region:   region,
+					City:     city,
+					Hostname: node.Hostname,
+					IP:       node.OpenvpnIP,
+				}
+				servers = append(servers, server)
+			}
+		}
+	}
+
+	if len(servers) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	sortServers(servers)
+
+	return servers, nil
+}