Add Raspberry Pi Pico port (#1232)

* Add Raspberry Pi Pico port
- Adds BACnet/IP with abstracted network layer
- Adds BACnet MS/TP support using RS485/UART
- Includes README with examples

* Add Pico 2 MS/TP demo, CI, and docs

ports/pico/mstimer_init.h

@@ -0,0 +1,25 @@
+/**************************************************************************
+ *
+ * Copyright (C) 2025 Testimony Adams <adamstestimony@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later WITH GCC-exception-2.0
+ *
+ *********************************************************************/
+
+#ifndef SYSTIMER_H
+#define SYSTIMER_H
+
+#include <stdint.h>
+#include "pico/time.h"
+#include "bacnet/basic/sys/mstimer.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+void systimer_init(void);
+
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
+#endif