add terrain occluded marker opacity option

[jacadzaca]

Attempts to solve #12123 by adding terrainOccludedMarkerOpacity option

Add new marker styling option terrainOccludedMarkerOpacity allowing the user to set the opacity of a marker that's behind 3D terrain

Launch Checklist

• [x] briefly describe the changes in this PR
• [ ] include before/after visuals or gifs if this PR includes visual changes
• [ ] write tests for all new functionality
• [x] document any changes to public APIs
• [ ] post benchmark scores
• [ ] manually test the debug page
• [ ] tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• [ ] tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• [ ] apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• [x] add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Allow to override marker opacity when occluded by terrain</changelog>

[CLAassistant]

All committers have signed the CLA.

[karimnaaji]

Continued in https://github.com/mapbox/mapbox-gl-js/pull/12258