add getFeatureInfo on WMSTileLayer

[fuzhenn]

Issue description

reported by 强瓦尔 in qq group chats.

To fetch feature info from WMS Server, implementation refers to:

https://github.com/CesiumGS/cesium/blob/main/Source/Scene/UrlTemplateImageryProvider.js#L791

Please provide a reproduction URL (on any jsfiddle like site)