Doc: Provide a way for broker plugins to get access to clients' TLS certificate information

The Artemis plug-in infrastructure, as it presently stands, provides no way for plug-ins to get access to TLS certificate information of clients that are making requests on the broker. Once a client is authentication, so far as the plug-in infrastructure is concerned, any further security-related decisions will be made by the broker itself.

In ActiveMQ it was possible for a plug-in to to get access to this information, so it was possible for a plug-in to control broker operation according to a customer security policy based on client certificates.

To provide comparable functionality in Artemis, it will be necessary of information derived from the Netty connector's SSL context to propagate all the way up to the plug-in layer.