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/AbstractBridge.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <Mesh.h>
+
+class AbstractBridge {
+public:
+  virtual ~AbstractBridge() {}
+
+  /**
+   * @brief Initializes the bridge.
+   */
+  virtual void begin() = 0;
+
+  /**
+   * @brief A method to be called on every main loop iteration.
+   *        Used for tasks like checking for incoming data.
+   */
+  virtual void loop() = 0;
+
+  /**
+   * @brief A callback that is triggered when the mesh transmits a packet.
+   *        The bridge can use this to forward the packet.
+   * 
+   * @param packet The packet that was transmitted.
+   */
+  virtual void onPacketTransmitted(mesh::Packet* packet) = 0;
+
+  /**
+   * @brief Processes a received packet from the bridge's medium.
+   */
+  virtual void onPacketReceived() = 0;
+};