[lib] Add type for thick thread avatar update request

Summary:
Some details in this Linear comment.

The "avatar update request" requires another, keyserver-agnostic type for encrypted images. The type can be used for setting image avatars for DM threads.

Test Plan: Flow, eslint. Used later in the stack

Reviewers: kamil, tomek, inka

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13284