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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,44 @@
+#ifndef CE45A952_0EE7_4D20_82CB_D42BF87F5B15
+#define CE45A952_0EE7_4D20_82CB_D42BF87F5B15
+
+#include <vanetza/security/sha.hpp>
+#include <array>
+#include <cstdint>
+#include <functional>
+#include <string>
+
+namespace vanetza
+{
+namespace security
+{
+
+using HashedId8 = std::array<uint8_t, 8>;
+using HashedId3 = std::array<uint8_t, 3>;
+
+HashedId3 truncate(const HashedId8&);
+
+HashedId8 create_hashed_id8(const Sha256Digest&);
+HashedId8 create_hashed_id8(const Sha384Digest&);
+
+std::string to_string(const vanetza::security::HashedId3&);
+std::string to_string(const vanetza::security::HashedId8&);
+
+} // namespace security
+} // namespace vanetza
+
+namespace std
+{
+// std::hash specialization for HashedId3
+template<> struct hash<vanetza::security::HashedId3>
+{
+    size_t operator()(const vanetza::security::HashedId3&) const;
+};
+
+/// std::hash specialization for HashedId8
+template<> struct hash<vanetza::security::HashedId8>
+{
+    size_t operator()(const vanetza::security::HashedId8&) const;
+};
+} // namespace std
+
+#endif /* CE45A952_0EE7_4D20_82CB_D42BF87F5B15 */