Bluetooth: Audio: Media control service

This commit adds the Media Control Service, and a dummy media player,
from the topic-le-audio branch.  This is a part of the upmerge of the
le-audio media control files.

This service has been developed and maintained over a couple of years
now, and is mature.  During the development it has passed both
IOP-testing and PTS qualification testing.

The commit is a pure copy of the files and content in the
topic-le-audio branch, with the following exceptions:

- files are in bluetooth/audio instead of bluetooth/host/audio, with
  some include paths updated as a consequence
- as a consequence, CMake files and Kconfig files updates are done in
  other locations
- copyrights have been updated

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>

subsys/bluetooth/audio/mcs_internal.h

@@ -0,0 +1,25 @@
+/* @file
+ * @brief Media Control Service internal header file
+ *
+ * Copyright (c) 2020 - 2021 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef ZEPHYR_SUBSYS_BLUETOOTH_HOST_AUDIO_MCS_INTERNAL_H_
+#define ZEPHYR_SUBSYS_BLUETOOTH_HOST_AUDIO_MCS_INTERNAL_H_
+
+#include <stdbool.h>
+#include <zephyr/types.h>
+#include <bluetooth/gatt.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int bt_mcs_init(struct bt_ots_cb *ots_cbs);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_SUBSYS_BLUETOOTH_HOST_AUDIO_MCS_INTERNAL_H_ */