bt2: do not require sink component's _graph_is_configured() method

The "graph is configured" method is not required when you implement a
component class in C, as it's not strictly needed to make the graph
runnable, so it should not be required in Python either.

A default _UserSinkComponent._graph_is_configured() method is added to
make it optional.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3ef4e3d159af61e2d06a0a47dddc1234aed93ae1
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1740
Tested-by: jenkins <jenkins@lttng.org>