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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,31 @@
+#pragma once
+#include <vanetza/security/hashed_id.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+class Certificate;
+
+/**
+ * Resolves a signing certificate's issuer by digest, typically backed by
+ * PKI-managed trust material rather than the peer-AT CertificateCache.
+ */
+class IssuerLookup
+{
+public:
+    virtual ~IssuerLookup() = default;
+
+    /**
+     * \param digest issuer's HashedId8
+     * \return pointer to the issuer certificate, or nullptr if unknown.
+     */
+    virtual const Certificate* find_issuer(const HashedId8& digest) const = 0;
+};
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza