[lib] Update thread-ops-utils to handle avatar field

Summary: Update convertRawThreadInfoToClientDBThreadInfo and convertClientDBThreadInfoToRawThreadInfo to accomodate avatar field.

Test Plan:

1. Set breakpoints and logged output of both functions in my local environment.
2. Made sure that we DON'T set avatar: null in RawThreadInfo if avatar is null on the SQLite side, we instead exclude it entirely.

Will also write some quick unit tests as a sanity check.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7300