Add @api stable annotations to ol.format

This PR adds stability markers to format-related options and methods.

Things to note:

- I'm not sure about what to make stable/experimental for the WFS and GML formats. So someone with more knowledge about these formats need to review my changes and propose/make new changes.

- Given that the WMS Capabilities format is currently the only non-feature related format, and given the current discussions about creating an OWS js [library](https://github.com/OSGeo/ows.js), I've decided to leave that format as "experimental".

Please review.