Add support for xyz map tiles format "https://tile.openstreetmap.org/{z}/{x}/{y}.png"

[alexiscatnip]

There is presently support for WMS and TMS map service formats.

But I would like to directly use some public apis which have the x/y/z/ format (osm tiles format?).

Examples: https://tile.openstreetmap.org/{z}/{x}/{y}.png

`var om2Basemap = new Cesium.UrlTemplateImageryProvider({ url: "../../basemap/Default/{z}/{x}/{y}.png", //url: 'https://maps-{s}.onemap.sg/v3/Default_HD/{z}/{x}/{y}.png', credit: 'OneMap (Default)' });

var custom = new Cesium.UrlTemplateImageryProvider({ url: '../../basemap/ortho_enhanced/tile/{z}/{x}/{y}.png', credit: 'SLA' });

var custom2 = new Cesium.UrlTemplateImageryProvider({ url: '../../basemap_hd/ortho_enhanced/tile/{z}/{x}/{y}.png', credit: 'SLA', maximumLevel: 20, minimumLevel: 4 });

var custom3 = new Cesium.UrlTemplateImageryProvider({ url: '../../MapServer3/tile/{z}/{x}/{y}.png', credit: 'SLA' });

var osmBasemap = new Cesium.createOpenStreetMapImageryProvider({ url: 'https://a.tile.openstreetmap.org/', credit: 'Open Street Map' });

var om2Satellite = new Cesium.UrlTemplateImageryProvider({ url: 'https://maps-{s}.onemap.sg/v3/Satellite/{z}/{x}/{y}.png', credit: 'OneMap (Default)' });

var om2Grey = new Cesium.UrlTemplateImageryProvider({ url: '../../basemap/Grey/{z}/{x}/{y}.png', credit: 'OneMap (Grey)' });

var om2Night = new Cesium.UrlTemplateImageryProvider({ url: '../../basemap/Night/{z}/{x}/{y}.png', credit: 'OneMap (Night)' });

var eld = new Cesium.UrlTemplateImageryProvider({ url: 'https://maps-c.onemap.sg/v3/ELD/{z}/{x}/{y}.png', credit: 'OneMap (Night)' });`

[j9liu]

Also requested in #262

[kring]

CC CesiumGS/cesium-native#614