Add new MQTT client example using natiu-mqtt

Add new MQTT client example using natiu-mqtt

examples/net/mqttclient/natiu/espat.go

@@ -0,0 +1,23 @@
+//go:build challenger_rp2040
+
+// +build: challenger_rp2040
+
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/espat"
+)
+
+var cfg = espat.Config{
+	// WiFi AP credentials
+	Ssid:       ssid,
+	Passphrase: pass,
+	// UART
+	Uart: machine.UART1,
+	Tx:   machine.UART1_TX_PIN,
+	Rx:   machine.UART1_RX_PIN,
+}
+
+var netdev = espat.New(&cfg)