common: add win32/wstring.h

common: add win32/wstring.h

Windows APIs heavily use wchar. ceph-dokan and rbd-wnbd
have some duplicated helpers that convert wstrings to/from
utf8 strings.

To avoid duplication and allow reusing those helpers, we're moving
them to common/win32/wstring.h.

We're using the "win32" subfolder because it's unlikely that this
will ever be used on other platforms.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>
(cherry picked from commit a47caa156a4e667d6b9ff7236cc9148ba37ea772)