Generate simple allowed values lists in XML and JSON schemas

[david-waltermire]

User Story:

As a Metaschema-based content creator, I need XML and JSON schemas that contain lists of allowed values for a given flag or field. This should be possible for some instances of allowed values that apply to the field or flag context they are defined on.

Goals:

• Add support for allowed value enumerations in JSON schema.
• Add support for allowed value enumerations in XML schema.

Dependencies:

None

Acceptance Criteria

• All goals above are supported.
• 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.}