Add sink.utils.counter

Add sink.utils.counter

A sink.utils.counter component counts the number of received
notifications (one count per notification type) and prints the counts
every 1000 notifications (total) by default and at the end. You can
use the `step` parameter to modify how frequent the component prints
the counts:

    babeltrace /path/to/trace -c sink.utils.counter -p step=5000

You can set `step` to 0 to print the counts only at the end:

    babeltrace /path/to/trace -c sink.utils.counter -p step=0

Notifications of which the type is not known (if this component is used
in future releases of Babeltrace 2) are classified under "Other
(unknown)", which is only printed if there's at least one unknown
notification.

You can use the `hide-zero` parameter (boolean) to hide the counts which
are 0.

The goal of this component class is to have a dummy sink component with
regular updates, for debugging and benchmarking purposes.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>