Allow FirebaseAuthConfig.RedirectUri to be configurable

[jthunter-continua]

The FirebaseAuthConfig.RedirectUri property is hardcoded to produce a redirect URL based on the AuthDomain with a static library-defined path component. In a testing tool I am working on I would prefer to redirect the URL to a non-prod landing page after a successful auth flow. This could easily be achieved if the RedirectUri could be specified in the config instead of being hard-coded.

I propose that the existing property be changed to allow it be optionally set. When not set, the current hard-coded Uri would still be used. But, if set explicitly then the provived URL would be used.