Allow to add multiple base URLs manually.

### Use case description
Following new question : https://github.com/google/ExoPlayer/issues/10191 , I am wondering if it would not be a good idea to allow Exoplayer to set multiple **base url** manually. Some services do not return the variants in one single manifest file, we could have those for example in different CDNS.
In this cases actually we need to initialize the player each time plus so we have an increase of VST (video start time).

### Proposed solution

I could see that **DashManifestParser** , read the multiple base URL that contains the manifest, creating a BaseUrl object.

So it should be possible to add a function let say. DashManifestParseraddBaseUrl(baseUrl: String) in this class.

This function will allow us to add manually our own CDN list.

If this is possible we will have all benefits of having multiple URLs in one single manifest file.