Add Size Ramp to Avatar macOS

[Saadnajmi]

Platform

• iOS
• macOS

Describe the feature that you would like added

The iOS Avatar has an MSFAvatarSize enum to create Avatars in specified sizes. The macOS Avatar does not do that, and currently can be configured to any point size by passing a CGFloat.
Let's add the same size values / enums to macOS, mostly so that in FluentUI React Native, they can have the same API.

What component or utility would this be added to

macOS Avatar

Have you discussed this feature with our team, and if so, who

No, mostly filing a bug so I can link the bug in comments of my PR code.

Additional context/screenshots