lib: keep plugin name, if any, in component class structure

lib: keep plugin name, if any, in component class structure

This is to make the plugin name available when having a component class
object in an upcoming error reporting API.

We don't keep the `bt_plugin` object itself because it's a component
class's owner, and a component class can outlive its containing plugin:
only the shared object data is kept alive in that case.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9cfbac32d88a6e7edac238b5a63b232695613b8c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1523
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>