Add NormalizedUserName / IUserNameNormalizer

UserManager will use normalized user name for all username operations on the store.
It will update the normalized user name, on Create/Update/SetUserName operations.
TBD: Should EF UserStore also add something in SaveChanges/Updates to do this as well?
Consider making this public so that customers that already have a UserManager instance and use it to convert a user name (e.g. coming from user input) into something they can pass to our APIs.