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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,40 @@
+#include "psid_ssp.hpp"
+#include <boost/algorithm/hex.hpp>
+#include <charconv>
+
+namespace vanetza
+{
+namespace pki
+{
+
+bool lexical_cast(const std::string& input, PsidSsp& permission)
+{
+    // split into the decimal PSID and the optional ':'-separated hex SSP.
+    const std::size_t colon = input.find(':');
+    const char* first = input.data();
+    const char* last = input.data() + (colon == std::string::npos ? input.size() : colon);
+
+    decltype(permission.psid) psid = 0;
+    auto [ptr, ec] = std::from_chars(first, last, psid);
+    if (ec != std::errc {} || ptr != last) {
+        // empty, contains a non-digit, or out of range
+        return false;
+    }
+
+    // found SSP part
+    if (colon != std::string::npos) {
+        try {
+            ByteBuffer ssp;
+            boost::algorithm::unhex(input.begin() + colon + 1, input.end(), std::back_inserter(ssp));
+            permission.ssp = std::move(ssp);
+        } catch (boost::algorithm::hex_decode_error&) {
+            return false;
+        }
+    }
+
+    permission.psid = psid;
+    return true;
+}
+
+} // namespace pki
+} // namespace vanetza