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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,78 @@
+#pragma once
+
+#include "keys.hpp"
+#include <boost/optional/optional.hpp>
+#include <boost/range/any_range.hpp>
+#include <string>
+
+namespace vanetza
+{
+namespace pki
+{
+
+/// \brief Lazy, single-pass view over canonical-hex credential names.
+using CredentialNameRange =
+    boost::any_range<std::string, boost::single_pass_traversal_tag, std::string, std::ptrdiff_t>;
+
+class CredentialStorage
+{
+public:
+    virtual ~CredentialStorage() = default;
+    virtual void store(const PublicKey&, const PrivateKey&) = 0;
+    virtual boost::optional<PrivateKey> fetch(const PublicKey&) = 0;
+    virtual bool discard(const PublicKey&) = 0;
+
+    /// \brief Lazy iteration over the canonical-hex name of every stored credential.
+    virtual CredentialNameRange list() const = 0;
+
+    /// \brief Discard a credential by its canonical-hex name; returns true iff one was removed.
+    virtual bool discard(const std::string& canonical_hex) = 0;
+
+    /// \brief Existence check by canonical-hex name; cheap, no key parsing.
+    virtual bool contains(const std::string& canonical_hex) const = 0;
+};
+
+/**
+ * \brief RAII handle that discards a credential on destruction unless commit() was called.
+ *
+ * Use to bind a freshly generated key to the lifetime of a protocol step:
+ * store now, drop the key if anything throws before the step succeeds.
+ */
+class ScopedCredential
+{
+public:
+    ScopedCredential(CredentialStorage& storage, PublicKey pub) : m_storage(&storage), m_public_key(std::move(pub))
+    {
+    }
+
+    ScopedCredential(CredentialStorage& storage, PublicKey pub, const PrivateKey& priv) : ScopedCredential(storage, pub)
+    {
+        m_storage->store(m_public_key, priv);
+    }
+
+    ~ScopedCredential()
+    {
+        if (m_storage) {
+            m_storage->discard(m_public_key);
+        }
+    }
+
+    // no copy
+    ScopedCredential(const ScopedCredential&) = delete;
+    ScopedCredential& operator=(const ScopedCredential&) = delete;
+    // no move
+    ScopedCredential(ScopedCredential&&) = delete;
+    ScopedCredential& operator=(ScopedCredential&&) = delete;
+
+    void commit()
+    {
+        m_storage = nullptr;
+    }
+
+private:
+    CredentialStorage* m_storage = nullptr;
+    PublicKey m_public_key;
+};
+
+} // namespace pki
+} // namespace vanetza