[flutterfire_ui] custom email validator

[komangnopha]

Hi, i am facing issue using flutterfire_ui, when i try to sign in using email that contains undescore or dash character, validator says "please provide valid email". Please help to resolve this issue. Thank You.

[lesnitsky]

@komangnopha this behavior will be changed after #7772 gets merged, so the underscore and dash character will be allowed.