Add original sort order as top fields ordering

[cal4]

Idea here is to add a option to the --order field. The usage would be to produce a document where the order of the to-document matches the order of the from-document as closely as possible.

This may not apply for disjoint formats, but could be useful when converting from OpenAPI 2 to 3 (and vice-versa). Results in less scrolling when comparing documents.

[diogonborges]

This would be very helpful