Integrate metaschema-xslt repository as submodule

[wendellpiez]

This repository now needs services now being offered in the metaschema-xslt repo - specifically the develop branch, but ongoing.

Specifically (in near term):

• InspectorXSLT production (dependency for OSCAL Inspector XSLT application #12)
• XSpec, XProc, bash and make patterns and infrastructure

In longer term, these pipelines would also support initiatives such as OSCAL in SaxonJS in NodeJS - XSLT-based schema production, data conversion, etc. (Assuming the utilities are also tested!)

Note: one reason for this is so oscal-xslt can be self-contained over metaschema instances specified dynamically, i.e. not hard wired to any version of OSCAL. Obviously when developing it is good and necessary to have OSCAL inputs including temporary OSCAL artifacts.

But this repository aims to be a submodule of OSCAL and thus must avoid that upstream dependency. The metaschema (version) for any process must be configurable dynamically.

For this Issue:

• Provide a support/metaschema-xslt submodule (develop branch in that repo)
• Provide CI/CD infrastructure and configuration (can borrow from metaschema-xslt)
• Provide a stub (Hello World) XSpec somewhere, invoked with a script under make
• Provide a PR addressing all the above, etc.

Additionally, avoid merge conflicts with WIP for #12 if/as possible.