Feature: Add Support for Custom auth-token validation

[ihipop]

What this PR does / why we need it

Add Support for Custom auth-token validation

• If openvpn.auth-token-user is enabled, the server will push the auth-token to the client, and finish quickly recover when the client is recovering from wakeup

• re-auth is also benefited from this PR, there will be a very quick re-auth even without enabling oauth2.refresh.enabled

• 20s delay for autorization after process restart #144 is also benefited from this PR if oauth2.refresh.enabled is disabled

Which issue this PR fixes

• fixes IOS OpenVPN Connect will disconnect after lock screen for about 20 seconds and can not auto re-connect after unlock #184
• fixes client doesn't sent auth-token if auth-user-pass is not defined OpenVPN/openvpn#296

Special notes for your reviewer

TODO: configurable AUTH-TOKEN lifetime checker

Particularly user-facing changes

Please ensure to remove auth-gen-token from your OpenVPN server configuration to make this work

Checklist

Complete these before marking the PR as ready to review:

• DCO signed
• The PR title has a summary of the changes
• The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR