src/cpp-common/bt2: wrap variant with selector field loc. FC API

src/cpp-common/bt2: wrap variant with selector field loc. FC API

This patch adds what's needed to the C++ libbabeltrace2 wrappers to
wrap:

* bt_field_class_variant_with_selector_field_borrow_selector_field_location_const()
* bt_field_class_variant_with_selector_field_location_integer_unsigned_create()
* bt_field_class_variant_with_selector_field_location_integer_signed_create()

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Id644771aa9537be21d73ba6cfe97a287c19f7240
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8019
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12702
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Simon Marchi <simon.marchi@efficios.com>