Addresses #14734.
This PR wires up `tunnel.go` to a `tailnet.Conn` via the new `/tailnet` endpoint, with all the necessary controllers such that a VPN connection can be started, stopped and inspected via the CoderVPN protocol.
`coder vpn-daemon run` will instantiate a RPC connection with the
specified pipe handles and communicate with the (yet to be implemented)
parent process.
The tests don't ensure that the tunnel is actually usable yet as the
tunnel functionality isn't implemented, but it does make sure that the
tunnel tries to read from the RPC pipe.
Closes#14735
Dean Sheather