[lib] Introduce *AvatarDBContent types

Summary:
Introduce basic [Emoji/Avatar]DBContent types that will be stringified and stored in the avatar columns of the users and threads tables.

These aren't in use yet, just putting up types so Ginsu and I have the interfaces agreed upon to make implementation easier.

Test Plan: NA,

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7097