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:

• [x] Implement a basic datatype and adapter (a subset of #7) for a basic metaschema type, such as a string type.
• [x] Research and choose an XML parser implementation with the features needed to parse complex metaschema documents
• [x] Create a bare metaschema parser with the capabilities needed to parse a basic metaschema
• [x] Create a metaschema to XML pipeline with the capabilities needed to write an equivalent metaschema xml definition
• [x] Create tests demonstrating the parser functionality (#2)

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

• [x] #19
• [x] #20
• [x] #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.