Bluetooth: HCI: Add support for user/vendor commands

Some Vendors can provide own subset of HCI commands/events, so provide them
possibility to write own dissectors and use them by "Decode As".

Change-Id: I87ff60ae12ac63ddd4b12b26951aa9f5143eabc6
Reviewed-on: https://code.wireshark.org/review/4166
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Evan Huus <eapache@gmail.com>