Add Node to Iterate Key/Value Pairs for Data Containers

Data containers are the GraphEx equivalent of dictionaries. Currently, they are a bit painful to use in practice. One way to make them easier to work with would be to have a node that retrieves the top level key/value pairs of a given data container. Similar to how you can iterate over a ditionary in python.

There is a node called 'Get Data Container Keys' that you can use for reference when writing this new node. Since we don't know the types of the values, perhaps we just cast them into strings.

Open to discussion on the best way to make this node and/or data container access better in general.

Start by locating the existing “Get Data Container Keys” node and the data container node implementations it uses. Read how nodes expose outputs and how container values are represented, then determine how top-level key/value pairs should be returned when value types are unknown. Done means the new iteration node works with a data container and has behavior consistent with the reference node.