[1pt] graph-prototype: Add API to (sub-)graph to access ports that are not connected or should be explicitly monitored

[ivan-cukic]

Ports can be disconnected for a few reasons:

• accidentally
• not meant to be connected
• the graph is meant to be used as a sub-graph, and those ports are meant to be connected to something later (e.g. monitoring purposes)

Proposal: [block&, port&] exported_input_ports() & [block&, port&] exported_output_ports()?

The implicitly/explicitly exported port names should be user-adjustable to give them more usage-specific names.

This task is meant to create API to list/access/define ports which are not connected (per-category)

To note (for UI): -> Annotate ports via 'Optional' tag/template parameter on Port<>.