cpp-common/vendor: add copy of wise_enum

cpp-common/vendor: add copy of wise_enum

Add a copy of wise_enum, a smart enum library that allows doing cool
things like getting the number of enumerators of an enum, iterating on
enumerators, and converting enumerators to string while effectively
keeping vanilla `enum`/`enum class` behind the scenes (no enum-like
class).

The version is 3.1.0, which corresponds to the git tag `3.1.0`, which
corresponds to this git commit:

  https://github.com/quicknir/wise_enum/commit/34ac79f7ea2658a148359ce82508cc9301e31dd3

Change-Id: I9b92d5cd012dee427cdd725c17eeae9f2a529cb0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12242
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>