Add file extension to individual tile URL

[ohrie]

My problem is, that I want to add a Cache-Control HTTP Header in my nginx proxy, because martin currently not yet (#749 ) supports the header. To select the tiles properly, nginx selects (analogous to Apache) the files via file extension (see example).
As GDAL writes in their docs: "typical extensions are .pbf, .mvt, .mvt.gz)" for Vector Tiles. Therefore I would propose to add the file extensions for the individual tiles and change the url in the TileJSON.

Therefore the TileJSON would look like this:

{
"tilejson":"3.0.0",
"tiles":["https://tileserver.com/streets/{z}/{x}/{y}.pbf"],
....
}

instead of
"https://tileserver.com/streets/{z}/{x}/{y}"

Mapbox specifies in their vector-tile-spec the file extension as .mvt.