cpp-common/bt2: add `bt2::SelfComponentClass`

cpp-common/bt2: add `bt2::SelfComponentClass`

A very straightforward wrapper of `bt_self_component_class`.

I didn't bother wrapping the individual
`bt_self_component_class_source`, `bt_self_component_class_filter`, and
`bt_self_component_class_sink` as they don't offer anything more.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ie96c61ba723d54ff720df9147279d7826dcf8fae
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11480
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>