field-class.hpp: Add CommonVariantWithSelectorFieldClass

field-class.hpp: Add CommonVariantWithSelectorFieldClass

This is useful to call the `selectorFieldPath()` method without knowing
if the variant field class has a signed or unsigned selector.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ib54bfa31d8f818430ec378c7c0e0c9258e20e450
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7511
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10761
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>