Allow to keep form validation when using custom save buttons

[JulienMattiussi]

Closes #4295

Issue

Form validation is never performed when using custom save buttons
(doing custom dava provider instead of dfault ra REST call)

Because when we overload handleSubmit or handleSubmitWithRedirect function, the form submission isn't performed

(And react-final-form doesn't offer a simple possibility to perform a form validation without a form submission.)

Solution

Add possibility to overload ra save function without overloading handleSubmit function

Use this

const SaveCustomButton = props => {
    const [create] = useCreate('posts');
    const redirectTo = useRedirect();
    const notify = useNotify();
    const { basePath } = props;

    const handleSave = useCallback(
        (values, redirect) => {
            create(
                {
                    payload: {
                        data: { ...values, average_note: 10 },
                    },
                },
                {
                    onSuccess: ({ data: newRecord }) => {
                        notify('ra.notification.created', 'info', {
                            smart_count: 1,
                        });
                        redirectTo(redirect, basePath, newRecord.id, newRecord);
                    },
                }
            );
        },
        [create, notify, redirectTo, basePath]
    );

    return <SaveButton {...props} onSave={handleSave} />;
};

Instead of this

const SaveCustomButton = props => {
    const [create] = useCreate('posts');
    const redirectTo = useRedirect();
    const notify = useNotify();
    const formState = useFormState();
    const { basePath, redirect } = props;

    const handleClick = useCallback(
        () => {
            create(
                {
                    payload: {
                        data: { ...formState.values, average_note: 10 },
                    },
                },
                {
                    onSuccess: ({ data: newRecord }) => {
                        notify('ra.notification.created', 'info', {
                            smart_count: 1,
                        });
                        redirectTo(redirect, basePath, newRecord.id, newRecord);
                    },
                }
            );
        },
        [formState, redirect, create, notify, redirectTo, basePath]
    );

    return <SaveButton {...props} handleSubmitWithRedirect={handleClick} />;
};

Doc

[fzaninotto]

To be PRed against next instead of master