[lib] Introduce new GenericUserInfoWithAvatar type

Summary:
I'll reuse this in later diffs, so factoring it out here.

Depends on D8044

Test Plan:
Flow, along with the testing I did for the whole stack:

1. ENS flow, both when user has an ENS avatar and when they don't
2. Setting an emoji avatar
3. Setting an image avatar via camera
4. Setting an image avatar via media library
5. Tested avatar update flow after being logged in to make sure I didn't regress anything

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8045