lib: pass config objects to component init methods

lib: pass config objects to component init methods

This patch introduces configuration objects that are passed to init
methods of components.  The goal is to make it possible to have
parameters that can be set during the component's init phase, and not
after.

At the moment, they are not used for anything, they are only there for
future use.  For that reason, we currently just pass the value NULL as
the config objects to the components.  This should be fine, since the
components should not do anything with the config objects for now.

Similarly, we have configuration Python objects passed to __init__
methods of _UserMessageIterator objects.  These configuration objects
are empty, but we can connect them with the corresponding SWIG pointer
once we actually add a method in them.

I added tests to verify that the config objects the components receive
have the right types.

Change-Id: If7568160e225c1eb8343eb212f8b4961f5472836
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2063
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>