Use MGLTileSet/MGLVectorSource instead of external style.json

[friedbunny]

After the tile set API stabilizes (https://github.com/mapbox/mapbox-gl-native/issues/7360), we should look at converting third_party_vector_style.json for the custom vector source example to a dynamically-created MGLTileSet/MGLVectorSource object (and eliminate the need for an external file).

/cc @ericrwolfe @boundsj @1ec5

[friedbunny]

Looks like we’ve already done a similar thing in the raster tileset example.

[captainbarbosa]

Yup, we should still do this.