[avatar] Allow links in `vaadin-avatar-group`

[abdonrd]

We usually use the vaadin-avatar with links like:

<a href="${urlForName('profile', { username: profile.username })}">
  <vaadin-avatar
    name="${profile.name}"
    img="${ifDefined(profile.picture)}"
    tabindex="-1"
  ></vaadin-avatar>
</a>

But if we want to use the vaadin-avatar-group... How can we add the link?
So that each avatar within the vaadin-avatar-group has its own link to the profile.