[core] Add unstable_ClassNameGenerator API

[siriwatknp]

close #28550

As mentioned in #28550 (comment). All of the global class name is generated from generateUtilityClass, so providing some global object to manipulate class name generation sounds like a good solution (since the class name does not need run-time changing).

• the docs is here: https://deploy-preview-29051--material-ui.netlify.app/guides/classname-generator/
• I put the API under utils folder so developers can import it like this

  import { unstable_ClassNameGenerator } from '@mui/material/utils';

• state classes eg. Mui-disabled does not have an impact from ClassNameGenerator

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• Fix hard-coded classname to use *Classes
  • Autocomplete
  • DialogContent
  • AvatarGroup

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• I have followed (at least) the PR section of the contributing guide.

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against b309859

[mnajdova]

Looks great! 👍

[oliviertassinari]

Are we sure it fixes the problem of @smora-gh? They seem to need multiple prefixes on the same page. The PR uses a global singleton, it will be unique for the whole page.

IMHO, the only real use case for a class name generator is white labeling. People that build a design system and don't want to see MUI mentioned in the DOM. I fail to envision other use cases.

[oliviertassinari]

A general rule, there is always blank space in place of the + or - in the git diff

   const theme = createTheme({