[keyserver] Update fetchServerThreadInfos(filter?: FetchThreadInfosFilter) to fetch special_role

Summary:
We want to update code that fetches from threads or roles to appropriately also fetch roles.special_role so we can include it in the return object. For now, I will leave in the r.special_role = ${specialRoles.DEFAULT_ROLE} AS is_default since I'm intermediately leaving in the isDefault property in RoleInfo. Towards the end of the stack, I will remove this check alongside the property as mentioned in the Linear task.

Part of ENG-5993

This diff handles thread.fetchers.js

Depends on D10106

Test Plan: Ran the query before and after to confirm the results are identical, except with the new special_role field

Reviewers: ginsu, tomek, rohan

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10107