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FLASHING.md says how to take a newer version of the colleague's tree (copy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/v3/persistence.cpp

@@ -0,0 +1,36 @@
+#include <vanetza/common/serialization.hpp>
+#include <vanetza/security/v3/persistence.hpp>
+#include <fstream>
+#include <iterator>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+Certificate load_certificate_from_file(const std::string& certificate_path)
+{
+    Certificate certificate;
+
+    std::ifstream certificate_src;
+    certificate_src.open(certificate_path, std::ios::in | std::ios::binary);
+    vanetza::ByteBuffer buffer(std::istreambuf_iterator<char>(certificate_src), {});
+    certificate.decode(buffer);
+
+    return certificate;
+}
+
+void save_certificate_to_file(const std::string& certificate_path, const Certificate& certificate)
+{
+    std::ofstream dest;
+    dest.open(certificate_path.c_str(), std::ios::out | std::ios::binary);
+
+    OutputArchive archive(dest);
+    serialize(archive, certificate);
+}
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza