Add CancellationToken to WebAuthenticator

[eth-ellis]

Description

It would be great to be able to cancel WebAuthenticator.AuthenticateAsync programatically using a CancellationToken.

Right now, TaskCancelledException is only thrown if the user taps the close button on the Chrome Custom Tab.

Public API Changes

public interface IWebAuthenticator
{
    Task<WebAuthenticatorResult> AuthenticateAsync(WebAuthenticatorOptions webAuthenticatorOptions);

    Task<WebAuthenticatorResult> AuthenticateAsync(WebAuthenticatorOptions webAuthenticatorOptions, CancellationToken cancellationToken);
}

Intended Use-Case

The intended use case would be to cancel the WebAuthenticator programmatically.

An example would be if the Chrome Custom Tabs Activity is closed due to deep linking.