Support for sending authentication tokens

[dsommers]

This plug-in should have support for sending 'auth-token' to clients, to avoid authentication failing on re-negotiations without the server using --auth-gen-token.

For more info:

Generates token: https://gitlab.com/openvpn/openvpn/commit/270dc91164013eb7ace34d7b098fa11a97aef847
Authenticates token: https://gitlab.com/openvpn/openvpn/commit/703c9784f4dcd4f77166201074c21c6ea4aeb033

This is the implementation inside the core OpenVPN for the --auth-gen-token, there are more related commits to this too. But these two is the core feature.

Any plug-ins supporting --client-connect should be able to write a configuration entry which can contain --push statements which are sent to the client. This authentication plug-in should make use of that feature to do a "push auth-token $RANDOM_STRING".

[update: point at proper commits]