[native] introduce Avatar component

Summary:
Introduced the Avatar component. For now, the avatar component only handles emoji avatars; however, in a subsequent diff we will extend this component to handle rendering image avatars.

This diff won't be landed until the avatars stack is all accepted

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Depends on D7053

Linear Task: https://linear.app/comm/issue/ENG-3139/build-avatar-component

Test Plan:
Please look at the screenshots to see the avatars rendered

Profile:

Large:

Small:

Micro:

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7055