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/surfshark/hosttoserver.go

@@ -0,0 +1,59 @@
+package surfshark
+
+import (
+	"net"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+type hostToServer map[string]models.SurfsharkServer
+
+func (hts hostToServer) add(host, region string) {
+	// TODO set TCP and UDP
+	// TODO set hostname
+	server, ok := hts[host]
+	if !ok {
+		server.Region = region
+		hts[host] = server
+	}
+}
+
+func (hts hostToServer) toHostsSlice() (hosts []string) {
+	hosts = make([]string, 0, len(hts))
+	for host := range hts {
+		hosts = append(hosts, host)
+	}
+	return hosts
+}
+
+func (hts hostToServer) toSubdomainsSlice() (subdomains []string) {
+	subdomains = make([]string, 0, len(hts))
+	const suffix = ".prod.surfshark.com"
+	for host := range hts {
+		subdomain := strings.TrimSuffix(host, suffix)
+		subdomains = append(subdomains, subdomain)
+	}
+	return subdomains
+}
+
+func (hts hostToServer) adaptWithIPs(hostToIPs map[string][]net.IP) {
+	for host, IPs := range hostToIPs {
+		server := hts[host]
+		server.IPs = IPs
+		hts[host] = server
+	}
+	for host, server := range hts {
+		if len(server.IPs) == 0 {
+			delete(hts, host)
+		}
+	}
+}
+
+func (hts hostToServer) toServersSlice() (servers []models.SurfsharkServer) {
+	servers = make([]models.SurfsharkServer, 0, len(hts))
+	for _, server := range hts {
+		servers = append(servers, server)
+	}
+	return servers
+}