[firebase_ui_auth] Add space between email and oauth widgets.

[bakua]

Hello,

can we have the option to set a space between the email Sign in button and the OAuth buttons, please? The current spacing just doesn't seem right.

[gazb23]

Unfortunately it looks even worse if you use OAuthButtonVariant.icon as there is no padding.

[timmolter]

Did anyone ever figure out how to add more space?