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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,35 @@
+#pragma once
+
+#include "certificate_storage.hpp"
+#include <filesystem>
+#include <memory>
+#include <string>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class SecurityModule;
+
+class CertificateFilesystemStorage : public CertificateStorage
+{
+public:
+    CertificateFilesystemStorage(std::shared_ptr<SecurityModule>, const std::filesystem::path&,
+        std::string extension = ".oer");
+    boost::optional<Certificate> fetch(const HashedId8&) const override;
+    void store(const Certificate&) override;
+    bool erase(const HashedId8&) override;
+    HashedId8Range list() const override;
+
+protected:
+    std::filesystem::path filename(const HashedId8&) const;
+
+private:
+    std::shared_ptr<SecurityModule> m_security;
+    std::filesystem::path m_root;
+    std::string m_extension;
+};
+
+} // namespace pki
+} // namespace vanetza