IVPN server data update code and ISP filter (#578)

- Use IVPN's HTTP API instead of their .zip file
- Unit tests for API and GetServers
- Paves the way for Wireguard
- Update server information for IVPN
- Add `ISP` filter for IVPN

internal/provider/ivpn/filter.go

@@ -11,6 +11,7 @@ func (i *Ivpn) filterServers(selection configuration.ServerSelection) (
 	for _, server := range i.servers {
 		switch {
 		case
+			utils.FilterByPossibilities(server.ISP, selection.ISPs),
 			utils.FilterByPossibilities(server.Country, selection.Countries),
 			utils.FilterByPossibilities(server.City, selection.Cities),
 			utils.FilterByPossibilities(server.Hostname, selection.Hostnames),