cpp-common/bt2: add component class creation methods

Add `create()` methods to `CommonSourceComponentClass`,
`CommonFilterComponentClass` and `CommonSinkComponentClass`, allowing
the user to create component classes at run time from some class that
derives from `UserSourceComponent`, `UserFilterComponent` and
`UserSinkComponent`.

The user component class (`UserComponentT`) must provide a static member
`name` of type `const char *`.  It can also provide static members named
`description` and `help`, also of type `const char *`.

Change-Id: I3bd550fd90a2a75d8154d29d00bf0bf4f9b1fd3e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11817