avatar-group: theme-override for overlap/gap

[OrbisK]

This function solves the problem (这个功能解决的问题)

height and width is customizable but not the overlap over avatars.

Expected API (期望的 API)

themeOverrides: GlobalThemeOverrides = {
  Avatar: {
    overlap: '3px'
  },
}

OR

themeOverrides: GlobalThemeOverrides = {
  Avatar: {
    gap: '-3px'
  },
}