Support `message` syntax and semantics to all constraint types

[aj-stein-gsa]

User Story:

As a Metaschema-enabled software developer, I want more flexible reporting for a variety of constraints not just expect, so I would like the ability to add a message to the following types of constraints:

• [x] Need to update the XML schema (in #42)
• [x] Need to update the Metaschema schema modules for constraints (in https://github.com/metaschema-framework/metaschema/pull/40)
• [ ] Update specification and publish website

Goals:

• [x] Syntax for the message in the identified constraints
  • [x] allowed-values
  • [x] has-cardinality
  • [x] index
  • [x] index-has-key
  • [x] is-unique
  • [x] matches
• [ ] Documented semantics of how they work differently from expect message usage, if applicable

Dependencies:

{Describe any previous issues or related work that must be completed to start or complete this issue.}

Acceptance Criteria

• [ ] All website and readme documentation affected by the changes in this issue have been updated. Changes to the 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.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}

[aj-stein-gsa]

@david-waltermire I just wanted to point out there a pretty good example of how the most efficient way to calculate model deviations with constraints for a large set of document instances (yes, plural) in OSCAL came up and I would much rather do it that way with index-has-key. However, knowing the user base of the constraints the output messages are hard to customize for downstream users who are not well-read on the Metaschema models. Example output here.

https://github.com/GSA/fedramp-automation/pull/736#pullrequestreview-2395336713

Is this something we can discuss later today?

[aj-stein-gsa]

Met with the maintainers and discussed this issue. They are not opposed moving forward.

• Need to update the XML schema
• Need to update the Metaschema schema modules for constraints
• Update specification and publish website
• Update implementations
  • metaschema-java maintainers is ready to implement an updated implementation to conform with a new spec after the next release (current upcoming release is pending as of this week).
  • metaschema-xslt is open to implement to be conformant with an updated spec, but there is no bandwidth at this time. It will be a medium to long-term goal.