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

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

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ic78e9a06ec86b3a1749c1763698408964f34ec27
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7323
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7290
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>