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.
Right. I'm happy to mark this format as "experimental", so we can decide to remove entirely in the future if nobody uses it.