cpp-common/bt2: add basic PluginSet and bt_plugin_find_all_from_dir wrappers

cpp-common/bt2: add basic PluginSet and bt_plugin_find_all_from_dir wrappers

Add a `bt2::PluginSet` class, wrapping `bt_plugin_set`.  It only
contains what I need for a test coming in a subsequent patch.

Add a `bt2::findAllPluginsFromDir` function, wrapping
`bt_plugin_find_all_from_dir`.

Change-Id: Ieca897dec5c08381deb473e701f7e41aee84e89c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11680
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>