Feature metaconstraints

[david-waltermire]

Committer Notes

This PR does two things:

1. Adds support for defining a Metaschema module's model as a Metaschema module.
2. Defines a new constraints format that can target based on a Metapth against the Metaschema module as a Metaschema module.

This involves adding support for usnistgov/metaschema#228, which means this PR depends on usnistgov/metaschema#470, which addresses this issue.

To full support the features of usnistgov/metaschema#228, the following needs to be completed.

• core: Add core XML model support for choice-group
• databind: Add code generation support for choice-group
• databind: Add XML serialization/deserialization support for choice-group
• databind: Add JSON/YAML serialization/deserialization support for choice-group
• schema-gen: Add XML schema generation support for choice-group
• schema-gen: Add JSON schema generation support for choice-group

As a stretch goal, it would be nice to add support for binding customization of the item type used for choice-group, to allow a base class or interface to be used instead of Object as the item item. This base class or interface would need to be implemented by each member of the choice-group.

• Add support for choice-group binding customization

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