[ButtonGroup] Allow override of the classname prop

[yipei1028]

ButtonGroup doesn't provide the functionality to override the look of children.

• The issue is present in the latest release.

• I have searched the issues of this repository and believe that this is not a duplicate.
  (Not 100% same but here comes a similar one as following: #16918, #16876)

Summary 💡

Should be able to be override the style by either

i. Using grouped property in

<ButtonGroup
        classes={{
          root: styles.menuCtrlBtn,
          grouped: styles.menuCtrlBtn
        }}
>

ii. Using className in children elements

<Button className={styles.menuCtrlBtn}>

Examples 🌈

My codesandbox: fancy-grouped-button

Motivation 🔦

[support]

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this issue appears to be a support request.

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[oliviertassinari]

See: https://material-ui.com/guides/interoperability/#css-modules. You need to get the CSS injection order right.