cpp-common: add `begin()` and `end()` to VariantWithIntegerSelector FC

cpp-common: add `begin()` and `end()` to VariantWithIntegerSelector FC

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ice31d346102258a6af593bd38fc3f123179482f4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7619
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10794
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>