[FEATURE] (super fast fix) add onSwitchSignUpAdditionalData as a parameter to the FlutterLogin widget

[nathantew14]

Is your feature request related to a problem? Please describe. In my app, I use the onSignup callback to make sure that the email entered by the user is unique (by checking with my database). However, I also use an additionalSignUpField for the user to to choose a username. This is a problem as the onSignup callback is only called after the user taps "confirm" on the additionalSignUpField page, so then the user would have to tap on the "back" button to go back and pick a different email.

Describe the solution you'd like This is easily fixed by adding a onSwitchSignUpAdditionalData parameter, which would require a Function that returns String?. It can be called at line 182 in the login_card file, and have the error be cleanly caught by the preexisting showError thing at line 195.