Add task ports.

This is a rough draft of what smtk::task::Port will look like.

• See the TestTaskPorts test for an example of its usage.
• There is no UI yet.
• FillOutAttributes implements an input and output port.
• Serialization/deserialization is only partially-complete (enough for the test to run, but not for project usage).
• The smtk::task::Manager operation observer does not yet invoke portDataUpdated() on tasks whose ports are marked modified.
• There is no operation to connect input to output ports.
• Design decisions on the Port lifecycle may change.

@jwilson1881 @rohit.krishna FYI.