Change the additionalSignupFields feature to getUserFieldsFn

[infomiho]

After we introduce the new auth models (#1080) which don't require mixing business logic fields and auth fields on the same User entity, the concept of "additional signup fields" won't make sense anymore. Since, there will be only "business logic user fields" but they are not "additional".

For Google & Github auth methods, we have the getUserFieldsFn which does exactly that - it specifies which User fields to set based on the all available data from e.g. Google like email.

We should convert our additionalSignupFields into getUserFieldsFn but it would receive all the signup form data and then the developer can decide what to put on the User object.

[sodic]

Closed by #1583