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

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

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ie442f80bf1d5b055f6707ff516a8094417ff6c1d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7553
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10792
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>