Api annotations required for the raster layers in ol3cesium project

This PR adds `@api` annotations required for the raster layer synchronization in the ol3cesium project.

Attribution done via OL3 or via Cesium? Here is implementation for Cesium.

getProjection on source to test 4326 and 3857 sources.
ol.TileCoord required for passing to tileUrlFunction
TileGrid exports getMinZoom() but not getMaxZoom()