Make alias input_type protected for join_node

[dbiswas2808]

Currently the public aliases exposed by flow_graph::join_node includes input_type. This is a bit confusing since join_node has multiple input ports and I believe other nodes(with multiple inputs) don't have the same issue. I have been leveraging substitution failure with input_ports_type and input_type and similarly for output_ports_type and output_type for implementing a higher level interface for flow_graph. AFAIT only join_node breaks this distinction. Also, having join_node expose input_type as a public alias could be confusing to users.