Address #173 by adding support for web redirect auth flows

This PR addresses #173 by adding a non-default option for redirect auth flows to flutter web apps.

This resolves an issue with being completely unable to login on iOS devices with home-screen installed PWAs made from a flutter app due to the authentication popup and the PWA web sandbox being isolated from each other.

It is possible to change the type of authentication by changing the option in the configuration after it is created.

My testing showed that an update to the MSAL library is required for this patch to work with the example application.

When using redirect flow, it is important to call login() again after the application is reloaded, following the completion of the redirect authentication flow, or the token acquisition will not have been completed. If the application triggers a login() whenever it first loads to acquire a token, this will be transparent to the user.

Closes #173

Closes #171