Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/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