Add support for Google SSO

[mvysny]

https://mvysny.github.io/vaadin-google-oauth/ - add additional documentation on how to have a user logging in both with a custom app password, and/or via a Google SSO.

Demo both ways:

1. Support for dual logins (user stored with the database as well)
2. Google SSO logins only - perhaps reuse the in-memory machinery

Even better, add a new module for Google SSO, which would depend on Google client API and would include the button implementation; the module would then document the Google SSO.

[mvysny]

Implemented in Vaadin Simple Security 1.1