Allow label position config in CheckboxGroupInput

[zhujinxuan]

Add labelPlacement props so that the CheckboxGroupInput can have labels on the top of checkboxes. It is useful when there is limited width for a checkbox group. For example, a weekday checkbox group squeezes in xs={4}.

I will continue on this PR for RadioGroupInput if you think this change is desirable. I find labelPlacement in RadioGroupInput looks weird. I will keep this PR only for CheckboxGroupInput.

[WiXSL]

It seems ok since we don't pass props to the FormControlLabel of the component.
Please, add labelPlacement in the table props of the component in the docs as well

[WiXSL]

Could you add screenshots of how it will look like?

[zhujinxuan]

It is an example based on example/simples/src/posts/PostEdit.tsx

                    <CheckboxGroupInput
                        source="notifications"
                        fullWidth
                        choices={[
                            { id: 12, name: 'Ray Hakt' },
                            { id: 31, name: 'Ann Gullar' },
                            { id: 42, name: 'Sean Phonee' },
                        ]}
                    />

                    <CheckboxGroupInput
                        label="Weekdays"
                        source="notifications"
                        choices={[
                            'Mon',
                            'Tue',
                            'Wed',
                            'Thu',
                            'Fri',
                            'Sat',
                            'Sun',
                        ].map(id => ({ id, name: id }))}
                        labelPlacement="top"
                    />

[slax57]

Thanks!