[Grid] Better support for theme.props

[oliviertassinari]

Closes #11665
Allow the usage of custom default values without a prop-type warning:

const theme = createMuiTheme({
  props: {
    // Name of the component ⚛️
    MuiGrid: {
      // The properties to apply
      spacing: 8,
    },
  },
});

[fondberg]

👍