Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/common/hook.hpp

@@ -0,0 +1,100 @@
+#ifndef HOOK_HPP_RNAM6XF4
+#define HOOK_HPP_RNAM6XF4
+
+#include <functional>
+#include <utility>
+
+namespace vanetza
+{
+
+/**
+ * Hook mechanism for realising extension points
+ */
+template<typename... Args>
+class Hook
+{
+public:
+    typedef std::function<void(Args...)> callback_type;
+
+    /**
+     * Assign a callback to hook, replaces previously assigned one
+     * \param cb A callable used as hook callback, e.g. lambda
+     */
+    void operator=(callback_type&& cb)
+    {
+        m_function = std::move(cb);
+    }
+
+    void operator=(const callback_type& cb)
+    {
+        m_function = cb;
+    }
+
+    /**
+     * Execute hook callback if assigned
+     * \param Args... various arguments passed to assigned callback
+     */
+    void operator()(Args... args)
+    {
+        if (m_function) {
+            // that's an arcane syntax, isn't it?
+            m_function(std::forward<Args>(args)...);
+        }
+    }
+
+    /**
+     * Reset previously assigned callback.
+     * No callback will be invoked when triggering hook after reset.
+     */
+    void reset()
+    {
+        m_function = nullptr;
+    }
+
+    /**
+     * \deprecated previous name of reset
+     */
+    void clear() { reset(); }
+
+private:
+    callback_type m_function;
+};
+
+/**
+ * Hook registry (non-callable view of a hook)
+ *
+ * Callbacks can be assigned to a hook via the corresponding registry,
+ * but the callback cannot be invoked through the registry.
+ */
+template<typename... Args>
+class HookRegistry
+{
+public:
+    using hook_type = Hook<Args...>;
+    using callback_type = typename hook_type::callback_type;
+
+    HookRegistry(hook_type& hook) : m_hook(hook) {}
+
+    void operator=(callback_type&& cb)
+    {
+        m_hook = std::move(cb);
+    }
+
+    void operator=(const callback_type& cb)
+    {
+        m_hook = cb;
+    }
+
+    void reset()
+    {
+        m_hook.reset();
+    }
+
+private:
+    hook_type& m_hook;
+};
+
+} // namespace vanetza
+
+#endif /* HOOK_HPP_RNAM6XF4 */
+