feat(GLAvatarLabeled): Add slot for meta data

What does this MR do?

Makes gitlab#235601 (closed) possible.

Adds a slot for meta data. This can be used for badges and user status. Specifically, I am working on converting the group members view from HAML to Vue (&4233 (closed)) and will be using this slot to display user status and badges next to the avatars.

Since this slot will mostly be used for badges I added a new example.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.