Integrate Spring Security with Google Accounts for GAE applications

General strategy is to create a a class that extends GenericFilterBean and can be added to the security:http element as a security:custom-filter with a postion="PRE_AUTH_FILTER". The filter would capture the users Gmail address using com.google.appenengine.api.users.UserServiceFactory. This value can be used to create a PreAutenticatedAuthenticationToken, which can later be utilized by a class the implements AuthenticationProvider, which in turn can be used by the AuthenticationManager.