Support picking for vector overlays

"Picking" refers to the ability to click on a specific location and lookup metadata from a tileset based on the location clicked. For tilesets we handle this by raycasting using the game engines' physics systems to find what tiles are under the cursor, and looking up metadata from those tiles. We need a way to lookup metadata from a `GeoJsonDocumentRasterOverlay` or `VectorTilesRasterOverlay`.

Start by reviewing the existing tileset picking flow, including its physics-system raycasting and metadata lookup, then compare it with GeoJsonDocumentRasterOverlay and VectorTilesRasterOverlay. The work is complete when a clicked location can identify the relevant overlay data and return its metadata for both overlay types.

Written by the indexing model from the issue text.