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- Fix: remove outdated and duplicate servers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

@@ -0,0 +1,9 @@
+package surfshark
+
+import "net/http"
+
+type roundTripFunc func(r *http.Request) (*http.Response, error)
+
+func (f roundTripFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return f(r)
+}