Implement serializers and provider for NodeItem subtypes

[nikitawootten-nist]

Following the design outlined in the document instance ADR entry, a serializer and provider for NodeItem subtypes must be fleshed out.

This workflow should include a serializer dynamically built off of an Assembly/Flag/Field definition or instance that can read XML and JSON in order to produce the appropriate Item class instance, or write XML and JSON from an existing Item class instance.

[nikitawootten-nist]

Sprint Debrief 1/31/2023

This item was just started a few days a go (last minute sprint inclusion) and will be continued next sprint.

[aj-stein-nist]

I am going to remove this from the sprint board. As Metaschema-related project, this one and others, pertain to the OSCAL work stream and sprint board, we have a pending meeting with Dave and Wendell on this front. I assume we can align the Metaschema vs. OSCAL work stream management outside of this and related tickets and track this somewhere else accordingly.