Add support for importing/exporting OpenAPI docs in YAML format

[ligreman]

Until 1.0.0-Beta1 version (I have not been able to test the most recent Beta versions) the import/export only accept JSON format.

Adding YAML format support is easy, just allow selecting this format and convert it to JSON with any javascript library. And add a new export option in YAML format, doing the inverse process (convert from json to yaml).

It does not require any modification in how Apicurio works with the information internally, only a little modification in the import and export flows.

Thanks.

[EricWittmann]

Because 1.1.0 is based on Apicurio Registry for the backend, it has much better YAML support. In that version, you can create a new API Design draft using a YAML file, and the application will simply remember that it is YAML. So you can edit it using the editor and all of that, but the content will remain YAML within Studio and Registry. So when you download it, you'll get a .yaml file instead of a .json file.

That feels like the right approach, but having an easy way to "download as JSON/YAML" would be easy, I agree.