OpenAPI specifications should be exported in a canonical form

[jsenko]

When user edits an Open API specification and creates a PR with the exported file, it's often hard to review the changes, because the maps in the exported JSON file may have been arbitrarily reordered. Therefore, it would be useful if the editor maintained a canonical ordering of the JSON keys, so the changes are more easily compared.

[jsenko]

This issue may be moved to https://github.com/Apicurio/apicurio-data-models if its more appropriate.