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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,35 @@
+#include <vanetza/security/v2/trust_store.hpp>
+#include <stdexcept>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+void TrustStore::insert(const Certificate& certificate)
+{
+    if (certificate.subject_info.subject_type != SubjectType::Root_CA) {
+        throw std::runtime_error("Only root certificate authorities may be added to the trust store");
+    }
+
+    HashedId8 id = calculate_hash(certificate);
+    m_certificates.insert(std::make_pair(id, certificate));
+}
+
+std::list<Certificate> TrustStore::lookup(HashedId8 id) const
+{
+    using iterator = std::multimap<HashedId8, Certificate>::const_iterator;
+    std::pair<iterator, iterator> range = m_certificates.equal_range(id);
+
+    std::list<Certificate> matches;
+    for (auto item = range.first; item != range.second; ++item) {
+        matches.push_back(item->second);
+    }
+    return matches;
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza