[keyserver] Update fetchServerThreadInfos to include avatar field

Summary:
Modify fetchServerThreadInfos to include avatar (if it exists) in response from fetchServerThreadInfos query.

Depends on D7332

Test Plan:
Set breakpoints and verified that avatar field was included in constructed object:

Verified that Redux state was updated to include avatar field (as result of updateThread => commitMembershipChangeset => rawThreadInfosFromServerThreadInfos => createUpdates):

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7333