Implement widget credentials request as per MSC1960

[MSC1960](https://github.com/matrix-org/matrix-doc/pull/1960) defines a way for widgets to request an OpenID token, which the widget can then verify the user with. This is implemented by matrix-react-sdk and used since Element Web v1.7.6 to support authenticated Jitsi widgets.

Start by reading MSC1960 and comparing the existing matrix-react-sdk implementation mentioned in the issue. Trace how Element Web uses the widget credentials request for authenticated Jitsi widgets, then identify the corresponding iOS widget entry point; done means iOS widgets can request an OpenID token that the widget can verify.

Written by the indexing model from the issue text.