Ability to establish known struct key order

[brandtkeller]

Is your feature request related to a problem? Please describe.

Order of the fields in tooling that consumes go-oscal was mentioned as a point of confusion for the generation of artifacts. Would like to test if it is possible to define ordered structs for the top level models.

Describe the solution you'd like

• Given Generation of a model
• When a new model is written to a file
• Then the order of the model fields is deterministic and ordered as intended

Describe alternatives you've considered

non-critical change

Additional context

Establish some error checking to ensure we capture drift when model is updated with new or different fields.

[brandtkeller]

Current Discovery - establishing struct key order for fields requires additional input data or hard-coded values.

hard-coding the order is not scalable and introduces a lot of potential for drift and additional work on each release and update of OSCAL schemas.

The JSON schema - for which go-oscal uses to-date has no precedence towards the order of keys (and the order which specified in the schema is not the preferred order).

Using the XSD with the JSON schema provided an option towards fully-generated types with keys ordered - but the differences between the XML schema and the JSON schema presented issues with regards to natively supporting both.

At this time I believe integrating further complexity for generate and the inclusion of XSD to not be aligned with greater library support of XML as a first-class format. As such we should defer this for review in work done in #12 .