fix(firebase_ui_auth): `sendPasswordResetEmail` should not be executed when `isLoading=true`

Description

Even when ForgotPasswordView has isLoading=true, sendPasswordResetEmail is executed the number of times the button is pressed. I don't see any advantage for users to receive more than one email.

In this PR, if isLoading is true, the mail is not sent.

Related Issues

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Breaking Change

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firebase / FirebaseUI-Flutter