Add bt_ctf_trace_get_stream_count() and bt_ctf_trace_get_stream()

Add bt_ctf_trace_get_stream_count() and bt_ctf_trace_get_stream()

With the two new functions you can access the streams of a trace. The
use case is for any component to be able to check if it has received a
"stream end" notification for each stream of a given trace and discard
anything associated with this trace if it's the case.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>