Add support for assigning references in importer API

[amykyta3]

Currently the importer API only allows building components with "regular" property values.
Add support for assigning instance and property references to component properties.

Some considerations to be aware of:

• References that involve array indexes may require more complex construction, since simply providing the terminal endpoints is not enough information.
  • Need an intuitive way for the user to be able to provide the lineage of array indexes as well.
  • Alternatively, do not support this yet?
• Will likely need to add a parent member to the Component class to facilitate construction of the internal ComponentRef object. I believe in most/all cases, parent shall never be deepcopied (copy by ref instead).
• This will likely be implemented as Importer.create_inst_reference() and Importer.create_prop_reference() methods.
  • Inputs are the source component, destination component, and source property name (for prop refs)
  • Returns a ComponentRef/PropertyRef object respectively.
• When assigning the reference object via Importer.assign_property(), check that a DPA is allowed.
  • Since prop refs imply a DPA, check that spec rules are not being violated inadvertently.