[lib] Introduce new avatar type for encrypted images

Summary:
This diff introduces new avatar type for blob uploads.
Existing codebase is updated to use this type in the next diff.

Test Plan: Tested later in the stack. CI (Flow) fails until next diff

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9167