Add bt_ctf_stream_class_create_empty()

Add bt_ctf_stream_class_create_empty()

The new bt_ctf_stream_class_create_empty() function creates a new
stream class object with its packet context, event header, and event
context field types set to empty structure field types.

This is more appropriate for source components which do not need the
default fields that bt_ctf_stream_class_create() creates.
bt_ctf_stream_class_create() is kept as is for backward compatibility
with pre-2.0 CTF writer.

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