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- Fix: remove outdated and duplicate servers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/remaining.go

@@ -0,0 +1,20 @@
+package surfshark
+
+import (
+	"github.com/qdm12/gluetun/internal/constants"
+)
+
+// getRemainingServers finds extra servers not found in the API or in the ZIP file.
+func getRemainingServers(hts hostToServer) {
+	hostnameToLocationLeft := constants.SurfsharkHostToLocation()
+	for _, hostnameDone := range hts.toHostsSlice() {
+		delete(hostnameToLocationLeft, hostnameDone)
+	}
+
+	for hostname, locationData := range hostnameToLocationLeft {
+		// we assume the server supports TCP and UDP
+		const tcp, udp = true, true
+		hts.add(hostname, locationData.Region, locationData.Country,
+			locationData.City, locationData.RetroLoc, tcp, udp)
+	}
+}