[lib] Introduce updateUserAvatar to user-actions

Summary: Remaining update_user_avatar scaffolding so we can hit endpoint from client. Nothing particularly interesting here.

Test Plan:
Able to hit keyserver endpoint from native client:

[will add screenshot shortly]

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7126