Support PROXY authentication mode in the UI.
PR 13114 added support for PROXY authentication mode to CDAP's backend and renamed the original mode of authentication to MANAGED.
The distinction between MANAGED and PROXY mode for the UI is that, in PROXY mode, the UI should be entirely unaware of any kind of authentication credential being passed around, whereas in MANAGED mode, the UI is an active participant in authentication (i.e. it reads a CDAP-issued auth token).
This means that, to support PROXY mode, we will need to stop sending the auth token in the websocket message payload (as well as make a few other changes).