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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,60 @@
+#include "distribution_centre.hpp"
+#include "http.hpp"
+
+namespace vanetza
+{
+namespace pki
+{
+
+DistributionCentre::DistributionCentre()
+{
+}
+
+void DistributionCentre::set_url(const std::string& url)
+{
+    m_base_query = HttpQuery::from_url(url);
+}
+
+boost::optional<CertificateTrustList> DistributionCentre::fetch_trust_list(const HashedId8& digest)
+{
+    HttpQuery query = m_base_query;
+    query.path += "/getctl/" + hexstring(digest);
+
+    HttpResponse response = http_get(query);
+    if (response.result() != boost::beast::http::status::ok) {
+        return boost::none;
+    } else if (response[boost::beast::http::field::content_type] != "application/x-its-ctl") {
+        return boost::none;
+    }
+
+    CertificateTrustList trust_list;
+    if (trust_list.decode(response.body())) {
+        return trust_list;
+    } else {
+        return boost::none;
+    }
+}
+
+boost::optional<CertificateRevocationList> DistributionCentre::fetch_revocation_list(const HashedId8& digest)
+{
+    // TS 102 941 v1.4.1 Annex D.2: GET <dc>/getcrl/<UPPERCASE-HEX-HashedId8>; reply application/x-its-crl.
+    HttpQuery query = m_base_query;
+    query.path += "/getcrl/" + hexstring(digest);
+
+    HttpResponse response = http_get(query);
+    if (response.result() != boost::beast::http::status::ok) {
+        return boost::none;
+    } else if (response[boost::beast::http::field::content_type] != "application/x-its-crl") {
+        return boost::none;
+    }
+
+    CertificateRevocationList crl;
+    if (crl.decode(response.body())) {
+        return crl;
+    } else {
+        return boost::none;
+    }
+}
+
+} // namespace pki
+} // namespace vanetza