support vector data in the ITilesetExternals interface or an analog

We now have support for rasterizing vector features, represented in GeoJSON, into image layers (#1163 on the native side). An obvious next step is to export vector data to Cesium Native clients as "vector tiles", so that a client can render the vector data or otherwise process it. Vector Tiles are a much-requested feature, and it's hard to know where to go with it. A possible approach would be to support tilesets that deliver vector data instead of meshes. If we choose a format for the vector features that is kind of "least common denominator,", then we can implement tileset providers for multiple formats: mvt, 3D tilesets with the MAXAR_content_geojson extension, ITwinFeatureService results...

A related question is how to style these features; see #1208.

Start by reviewing the ITilesetExternals interface and the vector-feature rasterization work referenced in #1163. Read the related styling question in #1208 and determine a supported vector-data representation and provider approach; done would require an agreed design rather than the issue's current proposal.

Written by the indexing model from the issue text.