Create tileset with the tiles.json path

[lilleyse]

For #3241

Currently when you create a Cesium3DTileset you pass in a url to the tileset folder. The code assumes the tiles json file is called tiles.json, which may not be true in all cases.

We should support options.url to be either a directory of the tileset or a path to the tiles.json file. In either case, this._url should stay the directory of the tileset and this._tilesJson should stay the path to the tiles.json file.

This will help clean up the tileset tests which can't reuse files due to this limitation.

[TomPed]

The url that is passed in to the tileset folder is the options.url?

[lilleyse]

Yeah options.url refers to the url of the tileset, which usually is the directory but in this PR it should also be able to be a path to a .json file.