Bluetooth: tester: Enable GAP discover feature for BR

Call `bt_br_discovery_cb_register` to register discovery callbacks.

Define a buffer br_adv_buf to report br discovery result.

Report device found event if discovery received callback triggered.

Start BR device discovery procedure if command `start_discovery` is
received.

Stop BR device discovery procedure if command `stop_discovery` is
received.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>