feat(GlAvatar): add responsive size support

What does this MR do?

Related to #1799 (closed)

Updates the GlAvatar size prop to accept an object. This allows for responsive avatars that are different sizes on different screens.

When providing an object to the size prop all breakpoints are min-width and you can specify a default key to set the size when no breakpoint is active. Example:

{
  default: 24,
  sm: 32,
  md: 48,
  lg: 96
}

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: gitlab!86586 (closed)
  • CustomersDot: GlAvatar not used
  • Status Page: GlAvatar not used
• Added the ~"component:*" label(s) if applicable.

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.