Implement widget credentials request as per MSC1960

[jaywink]

MSC1960 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.

[theCalcaholic]

FYI (and asking for confirmation of my comprehension of the issue): This also affects the sticker picker widget provided by a self-hosted dimension instance:

In web, you'd get this dialog when you use the sticker picker for the first time (per client):

If your click deny - or are using the android client which doesn't show the dialog above, you'll get this error in the sticker picker:

As I'm new to Matrix, I kindly ask to confirm whether I understand this correctly. :)
If that's the case, I'd also suggest to add the parity-with-web label

If not, I'll create a new issue.

[theCalcaholic]

I created a separate issue for my bug, since it appears to be unrelated: #2587