lib: create common base for bt_component_class_{source,filter}

lib: create common base for bt_component_class_{source,filter}

There are multiple spots which deal with message iterators, that have
duplicated code for source and filter components.  The code is the same,
except that one side deals with a bt_component_class_source and the
other with a bt_component_class_filter.

This patch introduce a common base,
bt_component_class_with_iterator_class, that holds the message iterator
class property.  The aforementioned code paths can then be deduplicated.

Change-Id: Ib2b42da4e77a0ab7faf94533684a7c1d665eb2e9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2744
Tested-by: jenkins <jenkins@lttng.org>