Introduce dco_get_peer_stats API and Windows implementation

Introduce dco_get_peer_stats API and Windows implementation

dco_get_peer_stats fetches stats for a single peer. This is mostly
useful in client mode. So far only Windows implements that.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221214211426.227-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25703.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 74d5ece4a035fbbd962ba5ea73c19118b82f8f45)