XSLT Metaschema support update and unit tests

[wendellpiez]

In the Metaschema repository, there is ongoing work to finalize the Metaschema model, and to harmonize its implementations going forward.

This is a tracking Issue for work on the XSLT M4 Metaschema implementation, making it ready to deploy supporting OSCAL assuming a complete v1.0 Metaschema implementation (for schema and converter generators).

Goals:

• Metaschema changes from #191 metaschema#195 with PR addressing it
• Glitch in (unused) feature on json-value-key metaschema#169 with PR addressing it
• Update XSLT M4 data converters to latest Metaschema design metaschema#212 with PR addressing it
• A commit to an OSCAL release branch of the updated metaschema (repo) submodule #1338

Related / Dependencies

Unit tests demonstrating functionality belong in both the metaschema-xspec and schema-generation folders of testing (in that repo). See usnistgov/metaschema#186

Potentially related: usnistgov/metaschema-xslt#19, usnistgov/metaschema-xslt#20

NOT IN SCOPE: extending functionality to new areas e.g. constraint testing

Acceptance Criteria

• Appropriate commits have been made in usnistgov/metaschema repository
• Commits include unit tests showing stability and protecting against regressions
• All OSCAL website and readme documentation affected by the changes in this issue have been updated. Changes to the OSCAL website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.