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/mullvad/string_test.go

@@ -0,0 +1,32 @@
+package mullvad
+
+import (
+	"net"
+	"strings"
+	"testing"
+
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Stringify(t *testing.T) {
+	servers := []models.MullvadServer{{
+		Country: "webland",
+		City:    "webcity",
+		ISP:     "not nsa",
+		Owned:   true,
+		IPs:     []net.IP{{1, 1, 1, 1}},
+		IPsV6:   []net.IP{{1, 1, 1, 1}},
+	}}
+	//nolint:lll
+	expected := `
+func MullvadServers() []models.MullvadServer {
+	return []models.MullvadServer{
+		{Country: "webland", City: "webcity", ISP: "not nsa", Owned: true, IPs: []net.IP{{1, 1, 1, 1}}, IPsV6: []net.IP{{1, 1, 1, 1}}},
+	}
+}
+`
+	expected = strings.TrimPrefix(strings.TrimSuffix(expected, "\n"), "\n")
+	s := Stringify(servers)
+	assert.Equal(t, expected, s)
+}