cpp-common: add `begin()` and `end()` to bt2::CommonEnumerationFieldClass

cpp-common: add `begin()` and `end()` to  bt2::CommonEnumerationFieldClass

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Id8b001611288121dd1adc1c3ae5bb698722b5bca
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7618
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10793
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>