cpp-common/bt2c: add format_as() template functions for wise enums

cpp-common/bt2c: add format_as() template functions for wise enums

Add some format_as() functions in the bt2 and bt2c namespaces that will
apply to all wise enums defined in that namespace.

Change-Id: I4952f062fc36e94925d2f0b093ac09d0fae6fdc0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12248
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>