ctf: add internal ctf_metadata_decoder_get_trace_class_uuid()

ctf: add internal ctf_metadata_decoder_get_trace_class_uuid()

This new internal function gets the trace class's UUID from the AST,
without having to create trace IR and CTF IR trace classes, which is a
much slower process (many allocations and checks).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I04d58c66a461219ca01bf120417d629d9a1b9d10
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1843
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>