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.c

@@ -0,0 +1,19 @@
+/**************************************************************************
+ *
+ * Copyright (C) 2025 Testimony Adams <adamstestimony@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later WITH GCC-exception-2.0
+ *
+ *********************************************************************/
+
+#include "mstimer_init.h"
+
+void systimer_init(void)
+{
+    //
+}
+
+unsigned long mstimer_now(void)
+{
+    return to_ms_since_boot(get_absolute_time());
+}