Add Coordinate Reference System ("crs")

[sfkeller]

Add "crs" (= Coordinate Reference System) as key. The values and if it should be mandatory has to be discussed. See also e.g. https://github.com/klokantech/tilejson-spec/tree/custom-projection/2.2.0

[perliedman]

See related https://github.com/mapbox/tilejson-spec/pull/6

[GretaCB]

Thanks for opening this ticket @sfkeller @perliedman . This proposed change will be considered as part of the v4 push https://github.com/mapbox/tilejson-spec/issues/35

[jo-chemla]

Hi there, has there been any decision regarding crs support for sources within the tilejson spec? Maptiler added support for it by forking the TileJSON 2.2.x spec