VPN support

[tbraun96]

VPN support should be added such that TUN/TAP adapts to a custom implementation of NetKernel, i.e., VPNNetKernel (VPNK). The VPNK needs to adapt to the TUN/TAP interface. The VPN "server" will be equivalent to the "globally reachable central server" already present within the Citadel Network's central server.

Client perspective (outgoing packets):
client packets -> Client VPNK -> forward to central server's VPNK over The Citadel Protocol -> forward to local tun/tap interface -> WAN

Server perspective (incoming packets):
WAN -> local tun/tap interface -> Server VPNK -> transmit packets over The Citadel Protocol to Client -> Client VPNK -> client receives packets

This would be a great first project for somebody interested in learning how to build ontop of the Citadel Protocol, and, is also interested in VPN technologies. I will be able to help extensively with this effort as needed.

[tbraun96]

Since this would require a custom implementation of a NetKernel, this should be a separate crate, potentially in this workspace.