Adds serial commands to get stats

- Added formatStatsReply, formatRadioStatsReply, and formatPacketStatsReply methods in MyMesh for both simple_repeater, simple_room_server, and simple_sensor.
- Updated CommonCLI to handle new stats commands.

src/helpers/StatsFormatHelper.h

@@ -0,0 +1,54 @@
+#pragma once
+
+#include "Mesh.h"
+
+class StatsFormatHelper {
+public:
+  static void formatCoreStats(char* reply, 
+                             mesh::MainBoard& board, 
+                             mesh::MillisecondClock& ms, 
+                             uint16_t err_flags,
+                             mesh::PacketManager* mgr) {
+    sprintf(reply, 
+      "{\"battery_mv\":%u,\"uptime_secs\":%u,\"errors\":%u,\"queue_len\":%u}",
+      board.getBattMilliVolts(),
+      ms.getMillis() / 1000,
+      err_flags,
+      mgr->getOutboundCount(0xFFFFFFFF)
+    );
+  }
+
+  template<typename RadioDriverType>
+  static void formatRadioStats(char* reply,
+                              mesh::Radio* radio,
+                              RadioDriverType& driver,
+                              uint32_t total_air_time_ms,
+                              uint32_t total_rx_air_time_ms) {
+    sprintf(reply, 
+      "{\"noise_floor\":%d,\"last_rssi\":%d,\"last_snr\":%.2f,\"tx_air_secs\":%u,\"rx_air_secs\":%u}",
+      (int16_t)radio->getNoiseFloor(),
+      (int16_t)driver.getLastRSSI(),
+      driver.getLastSNR(),
+      total_air_time_ms / 1000,
+      total_rx_air_time_ms / 1000
+    );
+  }
+
+  template<typename RadioDriverType>
+  static void formatPacketStats(char* reply,
+                               RadioDriverType& driver,
+                               uint32_t n_sent_flood,
+                               uint32_t n_sent_direct,
+                               uint32_t n_recv_flood,
+                               uint32_t n_recv_direct) {
+    sprintf(reply, 
+      "{\"recv\":%u,\"sent\":%u,\"flood_tx\":%u,\"direct_tx\":%u,\"flood_rx\":%u,\"direct_rx\":%u}",
+      driver.getPacketsRecv(),
+      driver.getPacketsSent(),
+      n_sent_flood,
+      n_sent_direct,
+      n_recv_flood,
+      n_recv_direct
+    );
+  }
+};