Bluetooth: Classic: L2CAP: implement ECHO REQ/RSP

Handle the ECHO request/response of classic L2CAP signaling packets.

Add the functions `bt_l2cap_br_echo_cb_register()` and
`bt_l2cap_br_echo_cb_unregister()` to register/unregister the ECHO
callbacks to monitor the ECHO REQ and RSP.

Add the function `bt_l2cap_br_echo_req()` to send the ECHO REQ through
classic L2CAP signaling channel.

Add the function `bt_l2cap_br_echo_rsp()` to reply the ECHO REQ
through the classic L2CAP signaling channel.

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