Jetty 12

In relation to #12659

The proxy uses secured transports, similar to the examples in org.eclipse.jetty.proxy.ClientAuthProxyTest

A websocket proxy must also be able to tag the ClientUpgradeRequest in order to be able to setup the ssl handshaking process to select a specific certificate on a per-request basis.

ClientUpgradeRequest upgradeRequest = ...
upgradeRequest.tag(new AliasTLSTag(...));
webClient.connect(endpoint, upgradeRequest, null);

Currently there is no way to do this with the websocket client AFAICT, but the underlying core client should be able to to it. So it is a matter of exposing such a method and copying the tag over to JettyClientUpgradeRequest in the connect.