Bluetooth: Classic: SDP: Improve SDP discovery response

In current implementation, the attribute of the registered SDP record
will not be included in responding packet if the attribute data length
is more than the MAX payload size(`MIN(SDP_MTU, sad->sdp->chan.tx.mtu)
- sizeof(struct bt_sdp_hdr)`). And it causes the failure of ssa or sa
discovery.

Improve the SDP server to support the case by extending the
continuation state length. A 4 bytes space is extended to store the
attribute offset.

In this way, the attribute can be transmitted in segments according
to the reaming space of the responding buffer.

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