[lib/native] lift AvatarSize type into lib

Summary:
This AvatarSize type is only accessible in the native codebase. However, we will need this for web as well so it makes sense to lift this type into lib. This diff just handles the lifting from native to lib, a subsequent diff will handle the actual usage of this type in web

Depends on D9364

Test Plan: flow

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9365