Support for web request headers when loading tilesets

Hi, we are trying to load tilesets that requires a specific access token. Right now it doesn't look like this is possible as you can only set the URL. Would be fantastic if `Cesium3DTileset` exposed some functionality for this.

Start by locating Cesium3DTileset and the code path that loads tilesets from their URL. Check how web requests are created, then define done as allowing callers to provide request headers such as an access token when loading a tileset; the issue does not name a test file.

Written by the indexing model from the issue text.