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.
does stable here mean that we will need to maintain this format? Does C2C have any business interest in this one? IIRC it came from one of Tom's personal projects or not?