MINOR: conn-stream: Add flags to set the type of the endpoint

MINOR: conn-stream: Add flags to set the type of the endpoint

This patch is mandatory to invert the endpoint and the context in the
conn-stream. There is no common type (at least for now) for the entity
representing a mux (h1s, h2s...), thus we must set its type when the
endpoint is attached to a conn-stream. There is 2 types for the conn-stream
endpoints: the mux (CS_FL_ENDP_MUX) and the applet (CS_FL_ENDP_APP).