Create a WebDriver Virtual Authenticator extension to allow automated testing

[nsatragno]

Currently there is no way for web developers to write automated tests that exercise the WebAuthn API. This problem is echoed on the WebAuthn WPT test suite - many of the tests assume mock authenticators that are not set up by any platform and so fail outright on all of them.

I propose creating a WebDriver extension defining an API that lets clients create and manipulate virtual authenticators that act transparently as any other authenticator on calls to the WebAuthn API. This would allow web developers to write tests and it would also allow rewriting the WPTs to use this API through testdriver.js.

The API at minimum should have commands to create and remove virtual authenticators, and to inject, inspect and remove credentials associated to them.

This would be similar to how the Generic Sensor API is automated.