feat(protonvpn): port forwarding support with NAT-PMP (#1543)

Co-authored-by: Nicholas Xavier <nicho@nicho.dev>

internal/natpmp/natpmp.go

@@ -0,0 +1,26 @@
+package natpmp
+
+import (
+	"time"
+)
+
+// Client is a NAT-PMP protocol client.
+type Client struct {
+	serverPort   uint16
+	initialRetry time.Duration
+	maxRetries   uint
+}
+
+// New creates a new NAT-PMP client.
+func New() (client *Client) {
+	const natpmpPort = 5351
+
+	// Parameters described in https://www.ietf.org/rfc/rfc6886.html#section-3.1
+	const initialRetry = 250 * time.Millisecond
+	const maxTries = 9 // 64 seconds
+	return &Client{
+		serverPort:   natpmpPort,
+		initialRetry: initialRetry,
+		maxRetries:   maxTries,
+	}
+}