CallAgentState interface

State only version of <xref:%40azure%2Fcommunication-calling%23CallAgent> except calls is moved to be a child directly of CallClientState and not included here. The reason to have CallAgent's state proxied is to provide access to displayName. We don't flatten CallAgent.displayName and put it in CallClientState because it would be ambiguious that displayName is actually reliant on the creation/existence of CallAgent to be available.