add Chapter 15 - runtime observer implementation

Chapter15/observer/hal/gpio/inc/gpio.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <cstdint>
+#include <functional>
+
+namespace hal
+{
+class gpio
+{
+public:
+    gpio(const std::function<void()> & on_press = nullptr);
+    void execute_interrupt_handler() const;
+
+    [[nodiscard]] virtual bool is_interrupt_generated() const = 0;
+    virtual void clear_interrupt_flag() const = 0;
+private:
+    std::function<void()> on_press_;
+};
+}; // namespace hal