ExternalGreyBoxBlock should support passing in input and output variables

[carldlaird]

Summary

Currently, ExternalGreyBoxBlock automatically creates input and output variables corresponding to the ExternalGreyBoxModel. It should allow a user to pass these in (or not) and, if provided, use the provided vardata objects instead of creating them.

We should provide support for getting the ordered list of inputs and outputs as vardata objects as well. Look at _BaseInputOutputBlockData for inspiration.

Rationale

Description

Additional information

[Robbybp]

I don't know what _BaseInputOutputBlockData is, but I implemented a version of this in #2022.