Bluetooth: SDP: Initial implementation of bt_sdp_discover API

Adds basic functionality to be able to trigger establishing outgoing
L2CAP channel to remote using as PSM SDP channel. The number of such
outgoing connections is limited by configured pool of connections.

Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>

Change-Id: Ie5428e5b1b5f5c57f473ee0adfd4621cd9fe6e3e
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>