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FLASHING.md says how to take a newer version of the colleague's tree (copy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