Bluetooth: Audio: Add seq_num and ts to bt_audio_stream_send

Added sequence number and timestamp to the bt_audio_stream_send
function. This allows an application to better
control the audio transmission, as it can schedule
one or more audio streams to send a buffer in a specific
SDU interval (ensuring that e.g. left and right is sent in
the same SDU interval).

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