Feat: rework Surfshark servers data (#575)

- Feat: `MULTIHOP_ONLY` variable
- Feat: `COUNTRY` variable
- Feat: `CITY` variable
- Feat: `REGION` variable, with retro-compatibility
- Feat: merge servers from API, zip and hardcoded hostnames
- Fix: remove outdated and duplicate servers
- Maint: faster update with fully parallel DNS resolutions

internal/updater/providers/surfshark/api.go

@@ -6,21 +6,43 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+
+	"github.com/qdm12/gluetun/internal/constants"
 )
 
+// Note: no multi-hop and some servers are missing from their API.
+func addServersFromAPI(ctx context.Context, client *http.Client,
+	hts hostToServer) (err error) {
+	data, err := fetchAPI(ctx, client)
+	if err != nil {
+		return err
+	}
+
+	hostToLocation := constants.SurfsharkHostToLocation()
+
+	const tcp, udp = true, true
+	for _, serverData := range data {
+		locationData := hostToLocation[serverData.Host] // TODO remove in v4
+		retroLoc := locationData.RetroLoc               // empty string if the host has no retro-compatible region
+		hts.add(serverData.Host, serverData.Region, serverData.Country,
+			serverData.Location, retroLoc, tcp, udp)
+	}
+
+	return nil
+}
+
 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"`
+	Region   string `json:"region"`
 	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"
@@ -37,7 +59,8 @@ func fetchAPI(ctx context.Context, client *http.Client) (
 	defer response.Body.Close()
 
 	if response.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("%w: %s", ErrHTTPStatusCodeNotOK, response.Status)
+		return nil, fmt.Errorf("%w: %d %s", ErrHTTPStatusCodeNotOK,
+			response.StatusCode, response.Status)
 	}
 
 	decoder := json.NewDecoder(response.Body)