Implement override-username

Implement override-username

This allow the server to set and override the username that is assumed
for the client for interaction with the client after the authentication.

This is especially intended to allow the of use auth-gen-token in
scenarios where the clients use certificates and multi-factor
authentication.

It allows a client to successfully roam to a different server and have
a correct username and auth-token that can be accepted by that server as
fully authenticated user without requiring MFA again.

The scenario that this feature is probably most useful
when --management-client-auth is in use as in this mode the OpenVPN
server can accept clients without username/password but still use
--auth-gen-token with username and password to accept auth-token as
alternative authentication. A client without a username will also not
use the pushed auth-token. So setting/pushing an auth-token-user
will ensure that the client has a username.

Github: OpenVPN/openvpn#299

Change-Id: Ia4095518d5e4447992a2974e0d7a159d79ba6b6f
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250311155904.4446-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31091.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>