Extract serial bridge into dedicated classes

This commit refactors the serial bridge functionality out of the `simple_repeater` example and into a more reusable, object-oriented structure.

An `AbstractBridge` interface has been introduced, along with a concrete `SerialBridge` implementation. This encapsulates all the logic for packet framing, checksum calculation, and serial communication, cleaning up the main example file significantly.

The `simple_repeater` example now instantiates and uses the `SerialBridge` class, resulting in better separation of concerns and improved code organization.

src/helpers/SerialBridge.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "helpers/AbstractBridge.h"
+#include <Stream.h>
+
+#ifdef BRIDGE_OVER_SERIAL
+
+/**
+ * @brief A bridge implementation that uses a serial port to connect two mesh networks.
+ */
+class SerialBridge : public AbstractBridge {
+public:
+  /**
+   * @brief Construct a new Serial Bridge object
+   * 
+   * @param serial The serial port to use for the bridge.
+   * @param mgr A pointer to the packet manager.
+   */
+  SerialBridge(Stream& serial, mesh::PacketManager* mgr);
+  void begin() override;
+  void loop() override;
+  void onPacketTransmitted(mesh::Packet* packet) override;
+  void onPacketReceived() override;
+
+private:
+  Stream* _serial;
+  mesh::PacketManager* _mgr;
+};
+
+#endif