Bluetooth: ISO: Introduce bt_iso_chan_send_ts

The bt_iso_chan_send function could take an optional
timestamp by using 0 as an indicator. The issue with
this approach was that a timestamp value of 0 is valid,
and could cause potential issue with syncing streams
in a group.

To fully support transmitting with and without timestamp,
bt_iso_chan_send_ts has been introduced, which is the only
function of the two (bt_iso_chan_send being the other) that
supports timestamps.

A new function, rather than adding a boolean to the existing,
was chosen as it simplifies the individual functions as well
as making it more explicit what the function does.

Since the bt_iso_chan_send function is used by LE audio, both
the BAP and CAP send functions have similarly been updated.
Likewise, all tests and samples have been updated to use the
updated function(s), and BT_ISO_TIMESTAMP_NONE has been
removed.

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