MEDIUM: conn-stream: Pre-allocate endpoint to create CS from muxes and applets

MEDIUM: conn-stream: Pre-allocate endpoint to create CS from muxes and applets

It is a transient commit to prepare next changes. Now, when a conn-stream is
created from an applet or a multiplexer, an endpoint is always provided. In
addition, the API to create a conn-stream was specialized to have one
function per type.

The next step will be to share the endpoint structure.