Add extension to keep track of original swagger version

[sebastien-rosset]

This is a proposed change to retain the original value of the "swagger" keyword when converting a OAS/Swagger 2.0 document to a 3.0 OAS document.

[gracekarina]

Hi @sebastien-rosset can you please update the branch so we can merge? thanks!

[sebastien-rosset]

Hi @sebastien-rosset can you please update the branch so we can merge? thanks!

Done. Just a thought, how about renaming the extension to x-original-openapi-version? In the future, there may be a similar use case when version 3 is converted to version 4, so it makes more sense to use openapi instead of swagger