Wireguard support for Mullvad and Windscribe (#565)

- `internal/wireguard` client package with unit tests
- Implementation works with kernel space or user space if unavailable
- `WIREGUARD_PRIVATE_KEY`
- `WIREGUARD_ADDRESS`
- `WIREGUARD_PRESHARED_KEY`
- `WIREGUARD_PORT`
- `internal/netlink` package used by `internal/wireguard`

internal/netlink/interface.go

@@ -0,0 +1,14 @@
+package netlink
+
+import "github.com/vishvananda/netlink"
+
+//go:generate mockgen -destination=mock_$GOPACKAGE/$GOFILE . NetLinker
+
+var _ NetLinker = (*NetLink)(nil)
+
+type NetLinker interface {
+	AddrAdd(link netlink.Link, addr *netlink.Addr) error
+	RouteAdd(route *netlink.Route) error
+	RuleAdd(rule *netlink.Rule) error
+	RuleDel(rule *netlink.Rule) error
+}