Bluetooth: Audio: Temporary Object Transfer client

This commit adds the Object Transfer client from the topic-le-audio
branch.  This is a part of the upmerge of the le-audio media control
files.  Media control depends upon Object Transfer for feature
completeness.

This client works, and has been used for IOP- and PTS-testing of the
media control server and client.  It does, however, need an overhaul
before being made "Zephyr official", so for now it will be kept as an
internal file to the le-audio code.

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
  an include path updated as a consequence
- a sa consequence, CMake files and Kconfig files updates are done in
  other locations
- In otc.c a superfluous semi-colon has been removed to please
  check_compliance
- Kconfig.ots has been renamed to Kconfig.otc, as there is only
  otc-content left in that file
- copyrights have been updated
- add TODO that these files are to be cleaned up and moved

The Object Transfer Client in this commit was originally written by
Leif Aschehoug <Leif-Alexandre.Aschehoug@nordicsemi.no>.
It has later been updated and maintained by
Emil Gydesen <emil.gydesen@nordicsemi.no> and
Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>

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

ots