bt2: check for _graph_is_configured method in user sink classes

bt2: check for _graph_is_configured method in user sink classes

Since a _graph_is_configured method is an essential method to implement
in any useful sink, check for the presence of that method when
instantiating a user sink component class in Python (that is, when the
Python class is created).  We already do that for the _consume method,
and I think it greatly helps the user writing a sink by telling them
what not to forget.

Change-Id: Ic8c3741b121eccc2857afac809521dd7213aa679
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1707
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>