Capture Subject into AuthenticationConfiguration

When an AuthenticationConfiguration is captured, it represents the identity being authenticated. Some mechanisms require a Subject to be established in order to retrieve the identity and credentials to use. The combination of these two facts implies that the AuthenticationConfiguration must encapsulate the Subject that was present at the time the configuration is realized by the ACCC. This Subject in turn must be propagated to the SASL authentication client (and later, possibly SSLContext to support KRB mechanisms).