Allow to set an URL from which the resumableIdentifier is fetched

[UniquePanda]

For better integration with backend processes, it should be possible to pass in a URL from which to fetch a resumableIdentifier from the server instead of creating it in the frontend.

[UniquePanda]

Nice idea, but for now we decided to not implement this.
While this would be easy to implement in resumable, the server side of this can be tricky, which is not worth it for us right now. 😄