This adds the ability to augment or override the NATS auth system.
A server will send a signed request to $SYS.REQ.USER.AUTH on the specified account. The request will contain client information, all client options sent to the server, and optionally TLS information and client certificates.
The external auth service will respond with an empty message if not authorized, or a signed User JWT that the user will bind to.
The response can change the account the client will be bound to.
Signed-off-by: Derek Collison <derek@nats.io>
