Add @api stable annotations for olx.control.DefaultsOptions

This follows up on #2564.

`olx.control.DefaultsOptions` currently includes the following options:

* `attribution`
* `attributionOptions`
* `rotate`
* `rotateOptions`
* `zoom`
* `zoomOptions`

Based on https://github.com/openlayers/ol3/issues/2236 I've marked `attribution`, `rotate` and `zoom` as stable, and left the corresponding `*Options` options experimental. In this way we can implement what's described in #2236 without breaking the API.

Please review.