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FLASHING.md says how to take a newer version of the colleague's tree (copy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/asn1/support/per_encoder.c

@@ -0,0 +1,35 @@
+#include "asn_internal.h"
+#include "per_encoder.h"
+
+int
+ignore_output(const void *data, size_t size, void *app_key) {
+    (void)data;
+    (void)size;
+    (void)app_key;
+    return 0;
+}
+
+int
+encode_dyn_cb(const void *buffer, size_t size, void *key) {
+    enc_dyn_arg *arg = key;
+    if(arg->length + size >= arg->allocated) {
+        size_t new_size = arg->allocated ? arg->allocated : 8;
+        void *p;
+
+        do {
+            new_size <<= 2;
+        } while(arg->length + size >= new_size);
+
+        p = REALLOC(arg->buffer, new_size);
+        if(!p) {
+            FREEMEM(arg->buffer);
+            memset(arg, 0, sizeof(*arg));
+            return -1;
+        }
+        arg->buffer = p;
+        arg->allocated = new_size;
+    }
+    memcpy(((char *)arg->buffer) + arg->length, buffer, size);
+    arg->length += size;
+    return 0;
+}