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FLASHING.md says how to take a newer version of the colleague's tree (copy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/security/exception.hpp

@@ -0,0 +1,28 @@
+#ifndef EXCEPTION_HPP_IY8LEMBQ
+#define EXCEPTION_HPP_IY8LEMBQ
+
+#include <stdexcept>
+
+namespace vanetza
+{
+namespace security
+{
+
+/// thrown when a serialization error occurred
+class serialization_error : public std::runtime_error
+{
+public:
+    using std::runtime_error::runtime_error;
+};
+
+/// thrown when a deserialization error occurred
+class deserialization_error : public std::runtime_error
+{
+public:
+    using std::runtime_error::runtime_error;
+};
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* EXCEPTION_HPP_IY8LEMBQ */