Implement ConfigureAuthorization, CheckAuthentication, and full auth
dispatch in NatsServer.Auth.cs; add HandleSignals in NatsServer.Signals.cs;
extend AuthHandler with GetAuthErrClosedState, ValidateProxies,
GetTlsAuthDcs, CheckClientTlsCertSubject, ProcessUserPermissionsTemplate;
add ReadOperatorJwt/ValidateTrustedOperators to JwtProcessor; add
AuthCallout stub; add auth accessor helpers to ClientConnection; add
NATS.NKeys package for NKey signature verification; 12 new tests pass.
8c380e7ca6