Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/tools/pki/credential_filesystem_storage.hpp

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "credential_storage.hpp"
+#include <filesystem>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class CredentialFilesystemStorage : public CredentialStorage
+{
+public:
+    CredentialFilesystemStorage(const std::filesystem::path& root);
+
+    void store(const PublicKey&, const PrivateKey&) override;
+    boost::optional<PrivateKey> fetch(const PublicKey&) override;
+    bool discard(const PublicKey&) override;
+    CredentialNameRange list() const override;
+    bool discard(const std::string& canonical_hex) override;
+    bool contains(const std::string& canonical_hex) const override;
+
+private:
+    std::filesystem::path build_key_path(const PublicKey&);
+    std::filesystem::path build_key_path(const std::string& canonical_hex) const;
+
+    std::filesystem::path m_root;
+};
+
+} // namespace pki
+} // namespace vanetza