Add size context for Avatar

[ogonkov]

That would be nice to have optional context, that could be used to sync Avatar sizes inside some common component, for example in AvatarStack

AvatarProvider.tsx

const AvatarProvider = ({size, children}: Props) => {
  return <AvatarContext.Provider value={{size}}>{children}</AvatarContext.Provider>;
}

Avatar.tsx

const Avatar = ({size = 'm'}) => {
  const avatarSize = useAvatarContext() || size;

  //
}