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FLASHING.md says how to take a newer version of the colleague's tree (copy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/v2/certificate_cache.hpp

@@ -0,0 +1,80 @@
+#ifndef VANETZA_CERTIFICATE_CACHE_HPP
+#define VANETZA_CERTIFICATE_CACHE_HPP
+
+#include <vanetza/common/clock.hpp>
+#include <vanetza/common/runtime.hpp>
+#include <vanetza/security/v2/certificate.hpp>
+#include <boost/heap/binomial_heap.hpp>
+#include <list>
+#include <map>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+/**
+ * CertificateCache remembers validated certificates for some time.
+ * This is necessary for certificate lookup when only its digest is known.
+ */
+class CertificateCache
+{
+public:
+    CertificateCache(const Runtime& rt);
+
+    /**
+     * Puts a (validated) certificate into the cache.
+     *
+     * \param certificate certificate to add to the cache
+     */
+    void insert(const Certificate& certificate);
+
+    /**
+     * Lookup certificates based on the passed HashedId8.
+     *
+     * \param id hash identifier of the certificate
+     * \param type type of certificate to lookup
+     * \return all stored certificates matching the passed identifier and type
+     */
+    std::list<Certificate> lookup(const HashedId8& id, SubjectType type);
+
+    /**
+     * Number of currently stored certificates
+     * \return cache size
+     */
+    std::size_t size() const { return m_certificates.size(); }
+
+private:
+    struct CachedCertificate;
+    using map_type = std::multimap<HashedId8, CachedCertificate>;
+
+    struct Expiry : public Clock::time_point
+    {
+        Expiry(Clock::time_point, map_type::iterator);
+        const map_type::iterator certificate;
+    };
+
+    using heap_type = boost::heap::binomial_heap<Expiry, boost::heap::compare<std::greater<Expiry>>>;
+
+    struct CachedCertificate
+    {
+        Certificate certificate;
+        heap_type::handle_type handle;
+    };
+
+    const Runtime& m_runtime;
+    heap_type m_expiries;
+    map_type m_certificates;
+
+    void drop_expired();
+    bool is_expired(const Expiry&) const;
+    void refresh(heap_type::handle_type&, Clock::duration);
+};
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza
+
+#endif /* VANETZA_CERTIFICATE_CACHE_HPP */