feat(GlAvatarLabeled): Add default slot

What does this MR do?

Related to gitlab#363510 (closed)

In gitlab#361986 (closed) we rearranged the user popover so it doesn't look so cramped. Currently it is using a custom markup and CSS to render the avatar and label and we would like to instead use GlAvatarLabeled to improve consistency.

This MR adds a default slot to GlAvatarLabeled so this is possible.

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: Adding an optional default slot shouldn't cause any specs to fail 🤞
• Added the ~"component:*" label(s) if applicable.