Bluetooth: AVDTP: Add AVDTP Pending Request

Added a Pending Request structure, this will keep
a track of the last sent AVDTP Singnalling Message.
This will be used to verify the response from the remote device
Memory will be allocated by the application.

Change-Id: Ic31df154b52ce9013e5039ab195a9651d1811a7a
Signed-off-by: Arun Jagadish <arun.jagadish@intel.com>