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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "CLI/Validators.hpp"
+#include "hexstring.hpp"
+
+namespace vanetza
+{
+namespace pki
+{
+
+class HexStringValidator : public CLI::Validator
+{
+public:
+    HexStringValidator() : Validator("HEX")
+    {
+        func_ = [](const std::string& input) {
+            if (is_valid_hexstring(input)) {
+                return "";
+            } else {
+                return "value must be an even, non-zero number of hex digits";
+            }
+        };
+    }
+};
+
+} // namespace pki
+} // namespace vanetza