Bluetooth: ISO: Add sn and ts to bt_iso_chan_send

Add two new parameters to bt_iso_chan_send:
sn: The packet sequence number which shall be incremeted
per SDU interval.
ts: An optional timestamp value used to synchronize SDUs.

The sequence number in the API uses a 32-bit value even though
the sequence number for the HCI command is 16-bit. This is to
properly handle wrapping of sequence numbers, which is much
easier to do with additional bits allocated.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>