sample-plugin: New plugin for testing multiple auth plugins

sample-plugin: New plugin for testing multiple auth plugins

This plugin allows setting username/passwords as well as configure
deferred authentication behaviour as part of the runtime initialization.

With this plug-in it is easier to test various scenarios where multiple
authentication plug-ins are active on the server side.

A test documentation was also added to describe various test cases and
the expected results.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20220313193154.9350-2-openvpn@sf.lists.topphemmelig.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23932.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 79a111c7e16d157278495cb5f4c52eab2229b68e)