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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,68 @@
+#include "certificate_filesystem_storage.hpp"
+#include "filesystem.hpp"
+#include "security_module.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/range/adaptor/filtered.hpp>
+#include <boost/range/adaptor/transformed.hpp>
+#include <boost/range/iterator_range.hpp>
+
+namespace vanetza
+{
+namespace pki
+{
+
+CertificateFilesystemStorage::CertificateFilesystemStorage(std::shared_ptr<SecurityModule> sec,
+    const std::filesystem::path& root, std::string extension) :
+    m_security(sec), m_root(root), m_extension(std::move(extension))
+{
+    std::filesystem::create_directories(m_root);
+}
+
+boost::optional<Certificate> CertificateFilesystemStorage::fetch(const HashedId8& id) const
+{
+    const std::filesystem::path path = filename(id);
+    if (std::filesystem::is_regular_file(path)) {
+        ByteBuffer buffer = read(path);
+        Certificate cert;
+        if (cert.decode(buffer)) {
+            return cert;
+        }
+    }
+
+    return boost::none;
+}
+
+void CertificateFilesystemStorage::store(const Certificate& cert)
+{
+    HashedId8 id = cert.calculate_hashed_id8(*m_security);
+    write(filename(id), cert.encode());
+}
+
+bool CertificateFilesystemStorage::erase(const HashedId8& id)
+{
+    return std::filesystem::remove(filename(id));
+}
+
+std::filesystem::path CertificateFilesystemStorage::filename(const HashedId8& id) const
+{
+    std::string filename = hexstring(id) + m_extension;
+    std::filesystem::path path = m_root / filename;
+    return path;
+}
+
+HashedId8Range CertificateFilesystemStorage::list() const
+{
+    auto rng =
+        boost::make_iterator_range(std::filesystem::directory_iterator(m_root), std::filesystem::directory_iterator()) |
+        boost::adaptors::filtered([this](const std::filesystem::directory_entry& e) {
+            return e.path().extension() == m_extension && std::filesystem::is_regular_file(e.path()) &&
+                   HashedId8::from_hexstring(e.path().stem().string()).has_value();
+        }) |
+        boost::adaptors::transformed([](const std::filesystem::directory_entry& e) {
+            return *HashedId8::from_hexstring(e.path().stem().string());
+        });
+    return HashedId8Range(rng);
+}
+
+} // namespace pki
+} // namespace vanetza