[keyserver] Add function for fetching RawMessageInfo for rows and their related messages

Summary:
issue: https://linear.app/comm/issue/ENG-3689/implement-fetching-edits-sidebar-source-and-reactions
This function takes rows from messages table, and returns RawMessageInfo for those rows and messages related, ie edits, reactions and sidebar_sources.
For sidebar_source, edit_message and reaction messages there are no sidebar_source, edit_message and reaction messages (user cannot react to a reaction message etc). So those messages are not passed to
fetchRelatedMessages function.

Test Plan: Called the function on some rows, checked that all related messages are returned (and the messages from the rows).

Reviewers: kamil, bartek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7584