Support transport-level authenticators

Currently each realm has to run it's own authenticator, this is broken design due to multiple reasons

The authenticator, when called by the router, doesn't know which realm it's authenticating an authid for
Designs where a single database decides which realm a specific user should be allowed into, don't work with Nexus

It would be very helpful if Nexus supported a transport-pevel authenticator, (aka single sign on) just like Crossbar does

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