Review metaschema-java's XML parser and implement an equivalent parser

[nikitawootten-nist]

As an OSCAL tool developer, I need the ability to serialize metaschema from valid, well-formed XML definitions into classes inheriting the metaschema-model-common core object model and back into XML.

This task can be decomposed into the following parts:

• Implement a basic datatype and adapter (a subset of Review metaschema-java's adapter and datatype design and identify work to implement #7) for a basic metaschema type, such as a string type.
• Research and choose an XML parser implementation with the features needed to parse complex metaschema documents
• Create a bare metaschema parser with the capabilities needed to parse a basic metaschema
• Create a metaschema to XML pipeline with the capabilities needed to write an equivalent metaschema xml definition
• Create tests demonstrating the parser functionality (Automated unit testing #2)

As of 8/1/22 this issue has the following sub-issues:

• Make the XML processor namespace-aware #19
• Implement parsing capabilities for all of the metaschema-common model definition classes #20
• Implement all metaschema-common model instance classes #21

[david-waltermire]

@nikitawootten-nist As we discussed, please note here what work remains to be completed in sprint 57. I am moving this issue to sprint 57 as a result.