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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,43 @@
+#include <vanetza/security/v2/subject_info.hpp>
+#include <vanetza/security/v2/length_coding.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+size_t get_size(const SubjectInfo& sub)
+{
+    size_t size = sizeof(sub.subject_type);
+    size += sub.subject_name.size();
+    size += length_coding_size(sub.subject_name.size());
+    return size;
+}
+
+void serialize(OutputArchive& ar, const SubjectInfo& sub)
+{
+    serialize(ar, sub.subject_type);
+    size_t size = sub.subject_name.size();
+    serialize_length(ar, size);
+    for (auto& byte : sub.subject_name) {
+        ar << byte;
+    }
+}
+
+size_t deserialize(InputArchive& ar, SubjectInfo& sub)
+{
+    deserialize(ar, sub.subject_type);
+    const std::uintmax_t size = deserialize_length(ar);
+    for (uintmax_t c = 0; c < size; ++c) {
+        uint8_t tmp;
+        ar >> tmp;
+        sub.subject_name.push_back(tmp);
+    }
+    return get_size(sub);
+}
+
+} // ns v2
+} // ns security
+} // ns vanetza