Add C++ API for Direct pairing

Add C++ API for Direct pairing

 Added C++ API for direct pairing feature
 Updated unittests for C++
 Added direct pairing sample
 Fixed Construct resource to '|' Secure flag instead of '&'

 Note: this change needs https://gerrit.iotivity.org/gerrit/#/c/8273/

Change-Id: I4a1cfc27206adf7308388406d0c8821aa46d87a8
Signed-off-by: Ashwini Kumar <k.ashwini@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/8277
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Chul Lee <chuls.lee@samsung.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>