Add @api stable annotations for the ol.layer namespace

This PR adds `@api stable` annotations to constructors and methods of the `ol.layer` package. I intentionally did not mark as stable the getters/setters related to hue, saturation, etc. Because these do not work with the Canvas renderer, which is our main (more capable renderer) currently.

Please review.
It would be nice to have useful docs for stable options, but I know you've already done a lot of work here.