add Chapter 15 - runtime observer implementation

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

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <array>
+#include <cstdint>
+
+#include <gpio.hpp>
+
+namespace hal {
+
+    struct gpio_interrupt_manager {
+        static void register_interrupt_handler(gpio * pin);
+
+        static void execute_interrupt_handlers();
+
+        static constexpr std::size_t c_gpio_handlers_num = 16;
+        static inline std::array<gpio*, c_gpio_handlers_num> gpio_handlers{};
+        static inline std::size_t w_idx = 0;
+    };
+};