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/vanetza/security/v3/certificate_cache.hpp

@@ -0,0 +1,64 @@
+#pragma once
+#include <vanetza/security/hashed_id.hpp>
+#include <vanetza/security/v3/certificate.hpp>
+#include <unordered_map>
+#include <unordered_set>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+/**
+ * CertificateCache stores validated v1.3.1 certificates for later lookup.
+ * Required for checking messages' signatures containing only a certificate digest.
+ */
+class CertificateCache
+{
+public:
+    /**
+     * Lookup certificate based on given digest
+     * \param digest certificate digest
+     * \return certificate matching digest
+     */
+    const Certificate* lookup(const HashedId8& digest) const;
+    const Certificate* lookup(const HashedId3& digest) const;
+
+    /**
+     * Store a (pre-validated) certificate in cache
+     * \param cert certificate
+     */
+    void store(Certificate cert);
+
+    size_t size() const { return m_storage.size(); }
+
+    /**
+     * Announce a station with a given certificate digest.
+     * \param digest certificate digest
+     * \return true if digest was not known before
+     */
+    bool announce(const HashedId8& digest);
+
+    /**
+     * Test if a certificate digest is already known, i.e. either
+     * its certificate is stored or at least the digest has been announced.
+     * \param digest certificate digest
+     * \return true if digest is known
+     */
+    bool is_known(const HashedId8& digest) const;
+
+private:
+    using CertificateMap = std::unordered_map<HashedId8, Certificate>;
+    using ShortDigestMap = std::unordered_map<HashedId3, CertificateMap::iterator>;
+
+    // TODO add bounded capacity and automatic removal of expired certificates
+    CertificateMap m_storage;
+    ShortDigestMap m_short_digests;
+    std::unordered_set<HashedId8> m_digests;
+};
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza