Bluetooth: BAP: Shell: Add support for multiple TX streams

Add support for transmitting on multiple streams, e.g.
with the simple `send` command or with the sine generator.

This extends the start and stop sine to take a "all"
paramter to start sending on all streams.

This also fixes an issue with the seq_num when multiple
streams are transmitting, since the timing may get delayed
which then delays the seq_num which are then scheduled too late.
The fix here is to only use the timer for the inital sequence
number when starting to send the sine way, and then increment it
per TX.

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