WebDriver commands for interacting with FedCM dialogs

Summary

This exposes several WebDriver commands to allow browser automation, such as automated testing, to interact with FedCM dialogs.

Specification: https://github.com/fedidcg/FedCM/pull/465
Firefox: Positive — Since this is not web-exposed, I do not think it makes sense to file a standards position request.
Safari: No signal — We have no signal from Safari for FedCM in general; as such, we have no signal for this part either
Web developers: Positive — At least one user is interested and sees the lack of this as a blocker to rolling out FedCM support
Tracking bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1441903