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FLASHING.md says how to take a newer version of the colleague's tree (copy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_decoder.h

@@ -0,0 +1,30 @@
+/*-
+ * Copyright (c) 2005-2017 Lev Walkin <vlm@lionet.info>. All rights reserved.
+ * Redistribution and modifications are permitted subject to BSD license.
+ */
+#ifndef	_PER_DECODER_H_
+#define	_PER_DECODER_H_
+
+#include "asn_application.h"
+#include "per_support.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct asn_TYPE_descriptor_s;	/* Forward declaration */
+
+/*
+ * Type of the type-specific PER decoder function.
+ */
+typedef asn_dec_rval_t(per_type_decoder_f)(
+    const asn_codec_ctx_t *opt_codec_ctx,
+    const struct asn_TYPE_descriptor_s *type_descriptor,
+    const asn_per_constraints_t *constraints, void **struct_ptr,
+    asn_per_data_t *per_data);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif	/* _PER_DECODER_H_ */