Implement the metaschema-cli query command

[aj-stein-nist]

Committer Notes

Closes #241.

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[aj-stein-nist]

I discussed with @david-waltermire-nist and I think to make this easily testable. I will have to adjust the existing tests first a bit, following my oscal-cli approach from here.

usnistgov/oscal-cli@4b6b205

/cc @Arminta-Jenkins-NIST

[aj-stein-nist]

Rebasing this off of the #271 branch until it lands, wanted to keep it separate from this code and just the relevant tests later.

[aj-stein-nist]

TODO Work:

• Add functionality to the CLI to support a document being passed in via STDIN not with an explicit file path.
• The Metaschema and document instance file path arguments should be URIable, not just local file paths. PRed separately to develop in Allow file or URI arguments to metaschema-cli #298 and this PR will rebase once merged.
• Add an --as=FORMAT command to support the output "converting" to desired output format of a node item.
• Render various forms of complex output, see challenges below.

Challenges:

1. What do we do if there is a collection of node items? A little more straightforward, join them into a single document dynamically for display purposes.
2. What do we do if there is a mixture of stringifiable values (atomic items) and node items? Re 1, this is a lot more complex than that because it now "two documents" you just dynamically join in a collection. Do we use a delimiter --- between results? Do we allow a CLI argument to change that delimiter at runtime? Food for thought.
3. It is unclear we should support a CLI runtime for "simple expressions only" for education and understanding, without a defined document instance and the module it conforms to it. It is not clear this is a primary goal or not, and it will potentially cause confusion (even A.J. forgets) because you need to process a document and module together. So where the criterion for simple enough or too complex is not clear at this time.
4. Maybe not in this PR, we are going to have to plan better work around error message and handling (that comes later).