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