[keyserver/lib] introduce fetch_community_infos endpoint

Summary:
This diff introduces the fetch_community_infos endpoint. Similar to the fetch_primary_invite_links we will call this endpoint whenever we open the community drawer on native or when the window becomes active on web

Depends on D11978

Test Plan: Called this endpoint later in my stack and confirmed that I was getting all the correct community infos

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: inka, ashoat, tomek

Differential Revision: https://phab.comm.dev/D11995