[lib][native][web] Introduce GlobalSearchIndexContext

Summary: This addresses ENG-9902.

Test Plan:

1. Confirm that render times have improved by patching this onto my test keyserver running on comm.domains, which is able to use the React profiler with my keyserver's production dataset
2. Make sure thread search still works

Reviewers: tomek, angelika

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13902