feat: add VPNsecure.me support (#848)

- `OPENVPN_ENCRYPTED_KEY` environment variable 
- `OPENVPN_ENCRYPTED_KEY_SECRETFILE` environment variable 
- `OPENVPN_KEY_PASSPHRASE` environment variable 
- `OPENVPN_KEY_PASSPHRASE_SECRETFILE` environment variable 
- `PREMIUM_ONLY` environment variable
- OpenVPN user and password not required for vpnsecure provider

internal/provider/vpnsecure/updater/servers.go

@@ -0,0 +1,57 @@
+package updater
+
+import (
+	"context"
+	"fmt"
+	"sort"
+
+	"github.com/qdm12/gluetun/internal/constants/vpn"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/common"
+)
+
+func (u *Updater) FetchServers(ctx context.Context, minServers int) (
+	servers []models.Server, err error) {
+	servers, err = fetchServers(ctx, u.client, u.warner)
+	if err != nil {
+		return nil, fmt.Errorf("cannot fetch servers: %w", err)
+	} else if len(servers) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	hts := make(hostToServer, len(servers))
+	for _, server := range servers {
+		hts[server.Hostname] = server
+	}
+
+	hosts := hts.toHostsSlice()
+
+	resolveSettings := parallelResolverSettings(hosts)
+	hostToIPs, warnings, err := u.parallelResolver.Resolve(ctx, resolveSettings)
+	for _, warning := range warnings {
+		u.warner.Warn(warning)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	if len(hostToIPs) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(servers), minServers)
+	}
+
+	hts.adaptWithIPs(hostToIPs)
+
+	servers = hts.toServersSlice()
+
+	for i := range servers {
+		servers[i].VPN = vpn.OpenVPN
+		servers[i].UDP = true
+		servers[i].TCP = true
+	}
+
+	sort.Sort(models.SortableServers(servers))
+
+	return servers, nil
+}