Bugifx: Allow style overrides for SimpleFormIterator

[andrico1234]

Resolves: #7629

Adding the following theme to the simple repo:

const theme = merge({}, defaultTheme, {
    components: {
        RaSimpleFormIterator: {
            styleOverrides: {
                root: {
                    border: '1px solid red',
                    [`& .${SimpleFormIteratorClasses.indexContainer}`]: {
                        border: '1px solid hotpink',
                    },
                    [`& .${SimpleFormIteratorClasses.action}`]: {
                        border: '1px solid blue',
                    },
                },
            },
        },
    },
});

Results in the following (Mondrian-inspired 👩🏼‍🎨🖌🎨) appearance 😅 :

[slax57]

Thanks! 🙂